The removal isn't directly related to the buyout/fork. Simple Gallery was taken off of F-Droid due to a dependency on the nonfree Google VR being discovered by IzzySoft¹². Fossify's fork has removed the dependent features to be compliant³.
F-Droid doesn't usually remove apps that aren't maintained, as far as I can tell. There are apps that haven't been updated in over a decade (Quill). Since F-Droid sorts by recency of release, they tend to just sink to the bottom of searches anyway.
Not getting updates doesn't make any app inherently bad. Possible bugs just won't get fixed. If an app is feature complete and everything works as they should there's no need to update it.
While I agree the apps shouldn't be removed, updates aren't just about features. Updates fix bugs, security holes, and improve performance. I'm notsaying these apps in particular have issues, though what app doesnt, just pointing it out in general.
The SMT apps after the buyout, are not necessarily bad either, it's up to the new owners whether they want to keep a FOSS version as advertisement, or not.
Their point regarding old and unmaintained apps seems a bit silly. Getting the same old and unmaintained app from the app's website isn't going to make it any safer. You're still going to need to switch to a different app/fork to get updates.
Everything was forked and should eventually end up on F-Droid, but most things haven't had a release yet. My understanding is that they're hoping to do everything right immediately, including having proper new branding and all the shared functionality from Simple Thank You.
The F-Droid versions of SMT apps are perfectly safe and shouldn't be going anywhere. (But if you have Google Play versions, I wouldn't trust those anymore, those are owned by ZipoApps now.)
Cool, thanks for the info! All of mine are f-droid and figured they'd be okay for now. Definitely looking to switch though but I'm lazy and prefer to do things through f-droid lol