Tldr They started charging 3rd party apps to use their API. The API that they had been using for ages and ages. They were shady about the timeline and their communication to devs was awful. So most 3rd party apps decided to end support as opposed to paying reddit.
Basically they wanted to make sure all users were seeing ads before their IPO.
I was a relay for reddit guy for ~10 years. Once the subscription update hit I uninstalled and moved to boost for lemmy.