Boost did stop working initially when Reddit moved to kill 3rd party apps, but it started working again pretty quickly if you were a mod (of ANY subreddit, even random ones that were created and instantly privated with zero members or content). Same for other 3rd party tools/apps.

Reddit sucks so much dick that any mod worth their salt on any major subreddit used/uses 3rd party apps for modding (because official Reddit was/is inadequate).

The API pricing change would've killed 3rd party mod tools, so a bunch of mods threatened to quit - which would (in theory, but probably actually not) fuck over a bunch of major subreddits right before Reddit's IPO. Reddit capitulated and allowed users (only if they were mods) to make API calls with 3rd party tools/apps. Presumably Reddit did this begrudgingly, as the best option to preserve value before their IPO.

Boost and other 3rd party tools were down for a relatively short amount of time (I think a few days, or maybe a week or two? I don't remember, but it felt quick) before coming back online. I've been using regular unpatched Boost ever since then, but it stopped working about a week ago. I thought it was just me, but apparently it's stopped working for everybody?

That said, I'm not convinced it's Reddit that's behind this, I think it may be Boost's developer, Ruben Mayayo. I get a 403 error now on Boost for Reddit, and the app - for the first time ever - showed me an ad for Boost for Lemmy. This can't be a coincidence, and Ruben must know that Boost for Reddit has stopped working. I'm curious if it's Reddit taking further action against 3rd party clients or if it's Boost's developer finally and decisively killing Boost for Reddit and notifying users of Boost for Lemmy. Maybe Ruben just wanted to finalize his split from Reddit and so he officially killed his app's access to Reddit's API?

Also yes, you could change the universal Boost API key to your own personal API key, and that would allow you to keep using it (or other 3rd party tools/apps).

Boost, and Sync I know can be made to work still for reddit with revanced patches. There are other reddit apps as well but I forgot which other ones is revanced supported.

Nope, you just had to have your own moderation API key or patch the APK file