MacOS – When we toggle between enable and disable safari extensions, where will the state go and sit on different mac os

It's primarily a matter of parsing the output of defaults read ~/Library/Safari/Extensions/extensions and formatting it in a user-friendly way. The result actually answers both of your questions:

defaults read ~/Library/Safari/Extensions/extensions | awk '
/Bundle Directory/ {
    split($0, t, /\"/);
    sub(".safariextension", "", t[4]);
    sub("-1", "", t[4]);
    bundle=t[4]
    e="disabled"
}
/Enabled/ {
    e="enabled";
}
/Hidden Bars/ {
    print bundle, "...", e
}' | sort --ignore-case

Creating a shell script out of it is left as an exercise to the reader.

Please note that this will most probably stop to work if Apple decides to change the format of ~/Library/Safari/Extensions/extensions

The "autocomplete" extension in the third link works for me.

Here is a step-by-step recipe on how to install it, for others who may run into this:

1. Launch Safari. In the menubar, click on Safari >> Preferences. Under the Advanced pane, tick the box labelled "Show Develop menu in menu bar" at the bottom.

2. In the Safari menubar, you should now see Develop. Click on Develop, then click on "Enable Extensions".

3. Download the extension from here: http://lapcatsoftware.com/downloads/autocomplete.safariextz

4. In your downloads folder, click on the file that you just downloaded. Safari will ask you whether you want to install this extension. Say yes.

Now you're good to go!