As of iOS 13.1, these URLs are back, and have been exhaustively documented on the MacStories website by Federico Viticci. Please note that the linked page is being actively updated, and contains other useful background and tips such as how to link to the settings for third party apps, so probably a good idea to follow the linke through to that site for the full info:
Settings URLs
iCloud
iCloud: prefs:root=CASTLE
iCloud Backup: prefs:root=CASTLE&path=BACKUP
Wireless Radios
Wi-Fi: prefs:root=WIFI
Bluetooth: prefs:root=Bluetooth
Cellular: prefs:root=MOBILE_DATA_SETTINGS_ID
Personal Hotspot
Personal Hotspot: prefs:root=INTERNET_TETHERING
Personal Hotspot ⇾ Family Sharing: prefs:root=INTERNET_TETHERING&path=Family%20Sharing
Personal Hotspot ⇾ Wi-Fi Password: prefs:root=INTERNET_TETHERING&path=Wi-Fi%20Password
VPN
VPN: prefs:root=General&path=VPN
Notifications
Notifications: prefs:root=NOTIFICATIONS_ID
Notifications ⇾ Siri Suggestions: prefs:root=NOTIFICATIONS_ID&path=Siri%20Suggestions
Sounds
Sounds: prefs:root=Sounds
Ringtone: prefs:root=Sounds&path=Ringtone
Do Not Disturb
Do Not Disturb: prefs:root=DO_NOT_DISTURB
Do Not Disturb ⇾ Allow Calls From: prefs:root=DO_NOT_DISTURB&path=Allow%20Calls%20From
Screen Time
Screen Time: prefs:root=SCREEN_TIME
Screen Time ⇾ Downtime: prefs:root=SCREEN_TIME&path=DOWNTIME
Screen Time ⇾ App Limits: prefs:root=SCREEN_TIME&path=APP_LIMITS
Screen Time ⇾ Always Allowed: prefs:root=SCREEN_TIME&path=ALWAYS_ALLOWED
General
General: prefs:root=General
General ⇾ About: prefs:root=General&path=About
General ⇾ Software Update: prefs:root=General&path=SOFTWARE_UPDATE_LINK
General ⇾ CarPlay: prefs:root=General&path=CARPLAY
General ⇾ Background App Refresh: prefs:root=General&path=AUTO_CONTENT_DOWNLOAD
General ⇾ Multitasking (iPad-only): prefs:root=General&path=MULTITASKING
General ⇾ Date & Time: prefs:root=General&path=DATE_AND_TIME
General ⇾ Keyboard: prefs:root=General&path=Keyboard
General ⇾ Keyboard ⇾ Keyboards: prefs:root=General&path=Keyboard/KEYBOARDS
General ⇾ Language & Region: prefs:root=General&path=INTERNATIONAL
General ⇾ Dictionary: prefs:root=General&path=DICTIONARY
General ⇾ Profiles: prefs:root=General&path=ManagedConfigurationList
General ⇾ Reset: prefs:root=General&path=Reset
Control Center
Control Center: prefs:root=ControlCenter
Control Center ⇾ Customize Controls: prefs:root=ControlCenter&path=CUSTOMIZE_CONTROLS
Display
Display: prefs:root=DISPLAY
Display ⇾ Auto Lock: prefs:root=DISPLAY&path=AUTOLOCK
Display ⇾ Text Size: prefs:root=DISPLAY&path=TEXT_SIZE
Accessibility
Accessibility: prefs:root=ACCESSIBILITY
Wallpaper
Wallpaper: prefs:root=Wallpaper
Siri
Siri: prefs:root=SIRI
Apple Pencil
Apple Pencil (iPad-only): prefs:root=Pencil
Face ID
Face ID: prefs:root=PASSCODE
Emergency SOS
Emergency SOS: prefs:root=EMERGENCY_SOS
Battery
Battery: prefs:root=BATTERY_USAGE
Battery ⇾ Battery Health (iPhone-only): prefs:root=BATTERY_USAGE&path=BATTERY_HEALTH
Privacy
Privacy: prefs:root=Privacy
Privacy ⇾ Location Services: prefs:root=Privacy&path=LOCATION
Privacy ⇾ Contacts: prefs:root=Privacy&path=CONTACTS
Privacy ⇾ Calendars: prefs:root=Privacy&path=CALENDARS
Privacy ⇾ Reminders: prefs:root=Privacy&path=REMINDERS
Privacy ⇾ Photos: prefs:root=Privacy&path=PHOTOS
Privacy ⇾ Microphone: prefs:root=Privacy&path=MICROPHONE
Privacy ⇾ Speech Recognition: prefs:root=Privacy&path=SPEECH_RECOGNITION
Privacy ⇾ Camera: prefs:root=Privacy&path=CAMERA
Privacy ⇾ Motion: prefs:root=Privacy&path=MOTION\
App Store
App Store: prefs:root=STORE
App Store ⇾ App Downloads: prefs:root=STORE&path=App%20Downloads
App Store ⇾ Video Autoplay: prefs:root=STORE&path=Video%20Autoplay
Wallet
Wallet: prefs:root=PASSBOOK
Passwords & Accounts
Passwords & Accounts: prefs:root=ACCOUNTS_AND_PASSWORDS
Passwords & Accounts ⇾ Fetch New Data: prefs:root=ACCOUNTS_AND_PASSWORDS&path=FETCH_NEW_DATA
Passwords & Accounts ⇾ Add Account: prefs:root=ACCOUNTS_AND_PASSWORDS&path=ADD_ACCOUNT
Mail
Mail: prefs:root=MAIL
Mail ⇾ Preview: prefs:root=MAIL&path=Preview
Mail ⇾ Swipe Options: prefs:root=MAIL&path=Swipe%20Options
Mail ⇾ Notifications: prefs:root=MAIL&path=NOTIFICATIONS
Mail ⇾ Blocked: prefs:root=MAIL&path=Blocked
Mail ⇾ Muted Thread Action: prefs:root=MAIL&path=Muted%20Thread%20Action
Mail ⇾ Blocked Sender Options: prefs:root=MAIL&path=Blocked%20Sender%20Options
Mail ⇾ Mark Addresses: prefs:root=MAIL&path=Mark%20Addresses
Mail ⇾ Increase Quote Level: prefs:root=MAIL&path=Increase%20Quote%20Level
Mail ⇾ Include Attachments with Replies: prefs:root=MAIL&path=Include%20Attachments%20with%20Replies
Mail ⇾ Signature: prefs:root=MAIL&path=Signature
Mail ⇾ Default Account: prefs:root=MAIL&path=Default%20Account
Contacts
Contacts: prefs:root=CONTACTS
Calendar
Calendar: prefs:root=CALENDAR
Calendar ⇾ Alternate Calendars: prefs:root=CALENDAR&path=Alternate%20Calendars
Calendar ⇾ Sync: prefs:root=CALENDAR&path=Sync
Calendar ⇾ Default Alert Times: prefs:root=CALENDAR&path=Default%20Alert%20Times
Calendar ⇾ Default Calendar: prefs:root=CALENDAR&path=Default%20Calendar
Notes
Notes: prefs:root=NOTES
Notes ⇾ Default Account: prefs:root=NOTES&path=Default%20Account
Notes ⇾ Password: prefs:root=NOTES&path=Password
Notes ⇾ Sort Notes By: prefs:root=NOTES&path=Sort%20Notes%20By
Notes ⇾ New Notes Start With: prefs:root=NOTES&path=New%20Notes%20Start%20With
Notes ⇾ Sort Checked Items: prefs:root=NOTES&path=Sort%20Checked%20Items
Notes ⇾ Lines & Grids: prefs:root=NOTES&path=Lines%20%26%20Grids
Notes ⇾ Access Notes from Lock Screen: prefs:root=NOTES&path=Access%20Notes%20from%20Lock%20Screen
Reminders
Reminders: prefs:root=REMINDERS
Reminders ⇾ Default List: prefs:root=REMINDERS&path=DEFAULT_LIST
Voice Memos
Voice Memos: prefs:root=VOICE_MEMOS
Phone
Phone: prefs:root=Phone
Messages
Messages: prefs:root=MESSAGES
FaceTime
FaceTime: prefs:root=FACETIME
Maps
Maps: prefs:root=MAPS
Maps ⇾ Driving & Navigation: prefs:root=MAPS&path=Driving%20%26%20Navigation
Maps ⇾ Transit: prefs:root=MAPS&path=Transit
Compass
Compass: prefs:root=COMPASS
Measure
Measure: prefs:root=MEASURE
Safari
Safari: prefs:root=SAFARI
Safari ⇾ Content Blockers: prefs:root=SAFARI&path=Content%20Blockers
Safari ⇾ Downloads: prefs:root=SAFARI&path=DOWNLOADS
Safari ⇾ Close Tabs: prefs:root=SAFARI&path=Close%20Tabs
Safari ⇾ Page Zoom: prefs:root=SAFARI&path=Page%20Zoom
Safari ⇾ Request Desktop Website: prefs:root=SAFARI&path=Request%20Desktop%20Website
Safari ⇾ Reader: prefs:root=SAFARI&path=Reader
Safari ⇾ Camera: prefs:root=SAFARI&path=Camera
Safari ⇾ Microphone: prefs:root=SAFARI&path=Microphone
Safari ⇾ Location: prefs:root=SAFARI&path=Location
News
News: prefs:root=NEWS
Health
Health: prefs:root=HEALTH
Shortcuts
Shortcuts: prefs:root=SHORTCUTS
Music
Music: prefs:root=MUSIC
Music ⇾ Cellular Data: prefs:root=MUSIC&path=com.apple.Music:CellularData
Music ⇾ Optimize Storage: prefs:root=MUSIC&path=com.apple.Music:OptimizeStorage
Music ⇾ EQ: prefs:root=MUSIC&path=com.apple.Music:EQ
Music ⇾ Volume Limit: prefs:root=MUSIC&path=com.apple.Music:VolumeLimit
Photos
Photos: prefs:root=Photos
Camera
Camera: prefs:root=CAMERA
Camera ⇾ Record Video: prefs:root=CAMERA&path=Record%20Video
Camera ⇾ Record Slo-mo: prefs:root=CAMERA&path=Record%20Slo-mo
Books
Books: prefs:root=IBOOKS
Game Center
Game Center: prefs:root=GAMECENTER
Best Answer
If the native QR code scanner in control center and the camera app aren’t sufficient, I use https://qrafter.com for this on iOS. The pro version is inexpensive and very useful.
It's possible to hand off URL - just that it's up to the developer to implement that.