If you’re talking about something for everyday controls, there are a couple of alternative apps, including sharptools and actiontiles. You still have to do some of the administrative things, like adding a new device, through the official app, but for day-to-day use, and even, in the case of sharptools, for rules creation, you don’t need to use the smartthings app.
The following discussion started out in the context of wall displays, but since these are mostly browser-based options, they work on a phone as well:
FAQ: Sharptools , actiontiles or native ST app for your wall panel? (2020)