Activating BitLocker will start a background process which encrypts all existing data. (On HDDs this traditionally is a long process as it needs to read and rewrite every partition sector – on self-encrypting disks it can be instant.) So when it is said that only newly written data is encrypted, that refers to the state immediately after BitLocker activation and is no longer true once the background encryption task finishes. The status of this process can be seen in the same BitLocker control panel window, and paused if necessary.
The Microsoft article needs to be read carefully: it actually talks about encrypting only used areas of the disk. They merely advertise this as having the biggest impact on fresh systems, where you don't have any data yet besides the base OS (and therefore all data will be "newly written"). That is, Windows 10 will encrypt all your existing files after activation – it simply won't waste time encrypting disk sectors which don't contain anything yet. (You can opt out of this optimization via Group Policy.)
(The article also points out a downside: areas which previously held deleted files will also be skipped as "unused". So if encrypting a well-used system, do a free-space wipe using a tool, and then let Windows run TRIM if you have an SSD, all before activating BitLocker. Or use the Group Policy to disable this behavior.)
In the same article, too, there is a mention of recent Windows versions supporting self-encrypting SSDs using the OPAL standard. So the reason why you don't see any background I/O may be because the SSD was internally encrypted from day one, and BitLocker recognized this and only took over the SSD-level key management instead of duplicating the encryption effort at OS level. That is, the SSD no longer unlocks itself on power-on but requires Windows do to so. This can be disabled via Group Policy, if you prefer the OS to handle encryption regardless.
Suspending BitLocker causes a plaintext copy of the 'master' key to be written directly to disk. (Usually this master key is first encrypted with your password or with a TPM.) While suspended, this allows the disk to be unlocked on its own – clearly an insecure state, but it allows Windows Update to reprogram the TPM to match the upgraded OS, for example. Resuming BitLocker simply wipes this plain key from disk.
BitLocker is not related to EFS – the latter works at file level, associating keys to Windows user accounts (allowing fine-grained configuration but making it impossible to encrypt the OS's own files), while the former works at whole-disk level. They can be used together, although BitLocker mostly makes EFS redundant.
(Note that both BitLocker and EFS have mechanisms for corporate Active Directory administrators to recover the encrypted data – whether by backing up the BitLocker master key in AD, or by adding an EFS data recovery agent to all files.)
Best Answer
You are disposing of a drive and want to ensure the data is not leaked.
If the data was stored in plain-text (i.e. not encrypted), you would have to ensure the data was 'shredded'. Just formatting the drive would not be sufficient; formatting doesn't overwrite the data, which is why there are 'unformat' utilities.
Instead, you'd consider a tool like DBAN. There are various erasure standards and as @Raystafarian points out, you can be all but sure the data is unrecoverable if you do a 7-pass shred. In truth, there are no reports of successful recovery after even a single pass on a modern mechanical hard drive, but it often pays to be conservative.
Matters are more complicated when considering SSDs. See this article (thanks to @Raystafarian for the link) for more information. SSDs remap blocks, which means you can overwrite all the data on the drive and some of the old data may have been remapped and not actually overwritten. DBAN doesn't handle SSDs.
Okay, but this isn't what you are doing. You have stored the data protected using Bitlocker and aren't planning on shredding or even overwriting the data at all. What you are suggesting is actually similar to how SSDs often implement 'secure erase'. They encrypt all data and then, during the 'secure erase', simply overwrite the block containing the key. Without the key, there's no practical way to recover the data.
This is true in your case, too. You cannot recover the data from a Bitlocker-encrypted drive without knowing the key, assuming no vulnerabilities in the Bitlocker implementation or in the underlying encryption algorithm. The fact that you know a great deal of the plain-text ("known Windows files or known data structures") is irrelevant; the encryption algorithm is strong despite known plaintexts.
In this case, against most adversaries, I'd suggest just doing a complete (as opposed to a quick) format. Even that shouldn't be necessary, but it's probably a good plan. Against a well-funded government or evil criminal empire? Well, you probably have other problems. And then I'd definitely be wiping the drive (using DBAN or a similar SSD-capable tool) and physically destroying the drive.