lmx that's definitely something an extension could implement. I don't think such an extension exist at this time unfortunately.
There is the Follow Tags Prompt extension that complements FoF Follow Tags with a prompt to the user, but it doesn't include any of the other notifications built into Flarum.
lmx there were less than 10 users with preferences data and there rest were NULL
I assume you figured this out, but to confirm NULL
values are the users who never customized their preferences, so it uses whichever value is configured as default by Flarum or an extension. Once the value has been customized, it's added to the database and the saved value will be used, even if the default changes in the future.