I'm not sure the author of Simple Social has received a letter yet, but apparently he's somewhat expecting to and has already removed the apps from availability and ended support. I found out about it because the final update (or an in-app news function?) popped up a notice that support was ending and the apps were being removed due to Facebook actions.
The author of Swipe apparently had his personal Facebook and Instagram accounts permanently banned with no advance notice around the same time as receiving said letter, and has also removed the apps from Google Play. Perhaps by having done so he can convince them to at least have access to accounts (apparently used for contact with family).
Maki is apparently another such tool which has been removed.
Friendly appears to still be available, and IIRC is the only one I know of that's also on iOS. At least on iOS it doesn't completely remove Sponsored posts, just collapses them.
SlimSocial is an open source option available through F-Droid and (currently) the Play Store along with its github page. Not sure how actively developed it is, the XDA thread for it and its FB page appear pretty quiet.
Frost-for-Facebook is another open source option available through F-Droid or its own github page. Not in the Play store and apparently has not been.
And as noted, on Apple devices there seem to be fewer options based on a very cursory search - Friendly was the only one that looked significant.
For those not familiar with the apps, they're all basically wrappers (custom CSS, maybe some tweaked HTML and JS) around Facebook's mobile website. They may lack features like notification of responses, etc and Messenger messages, but they're also much lighter-weight than the official apps, don't want access to your contacts, camera (in at least some cases), etc.
