The recent changes to the API in 0.50 look good. The number of iteration passes really shows. Out of the available open source editor frameworks, Slate is probably most similar to Notion’s internal editor system. If I had to rebuild on a framework tomorrow, it’d be Slate or ProseMirror. Still if I used Slate it would be with the expectation that I’d end up owning an aging fork of a forgotten version at some point.
That's my experience. I own a fork of Slate and Slate plugins that I've been doing my best to maintain, as well as a wrapper: https://github.com/roast-cms/french-press-editor. My project is small enough not to have bandwidth to upgrade Slate or switch to another editor but popular enough to keep alive and maintain two years past the version release I solidified. The implementation I have isn't very buggy, I've used it almost every day and have over a hundred users who successfully used it as well. There was only one time when a serious bug surfaced, the rest being minor inconveniences. This is the implementation: https://www.analog.cafe/write/draft