Hb1hf "Are devs equally likely to give up on maintaining their apps on a store with 3 billion users, one with 200 thousand users or one with 1000 users?"
To me, this question could be argued in all sorts of ways with absolutely no way of answering clearly. For example, I could argue that devs who publish their app on Accrescent, are more likely to keep up the maintenance needed for Accrescent, rather than for the Play Store. Because I would say, devs who wish to publish on Accrescent are more likely to be invested in this app store and the goals of the team behind it. Not just the security and privacy enhancements for users, but for the devs that publish there too. Even the fact that Accrescent is far less well known than the Play Store implies that the devs who publish there are invested in it as they were either searching for a store that would meet their needs, or probably heard of it because they are involved with, or following, projects with similar aims, such as increasing privacy and freedom for users etc, etc...
Since both mine, and your, points are pretty much based on assumptions about the motivations of the devs who publish on both app stores, I reckon... Well basically, it's pointless to speculate on.
As to your main question, I have personally had apps that were installed by the play store, decide to no longer allow installations on devices that don't pass Play Integrity, despite remaining compatible.
In these instances I do not recall being notified, but when I went to the app page in the Play Store, it did say, underneath the uninstall button, that "this app is no longer compatible" (I can't remember for sure, but I think it might have said it was no longer compatible with my device by choice of the developer, or something similar).
In one case, the app in question was Netflix, and I decided to install Aurora Store to check if they were actually enforcing the Play Integrity, and they weren't. Ridiculously, the app worked absolutely fine when installed by Aurora (by spoofing the device as a Pixel with the stock OS).
I don't know if it would be different if the app was abandoned by the dev. Perhaps Google would respond differently. I mean, isn't that what "Play Protect" is for?
I would also be interested to know if Accrescent would have some way of notifying the user if an app was abandoned/stopped being compatible etc...