as sibling commented, i look at the Automatic-1111 stable diffusion "webui" as being exactly like you describe, where the "default" service is built in, but the API seems relatively straightforward enough that people have build electron apps and android apps that can talk directly to the python service behind. one of these days i'll actually code a useful mobile UI for my servers. Until then, the reflowing webUI works "ok enough" on mobile.

another one that i'd like to think has an open enough API is mastodon/fediverse, assuming they actually adhere to the activitypub spec, it should (should) be relatively straightforward to write a UI on top of the API. And it seems ok, there's quite a number of mastodon "apps", but less for stuff like misskey or pleroma.

I'm sure there are other contemporaneous examples, but suffice to say that some developers still care about this, today!