I do have a UI Manager that handles standardized text (no special formatting and standard headers, icons, etc). The reason I want to have long rich text for other things is that the formatting is not standardized. Size, location, and number of headers... inline icons and special formatting... things like that where just having a set of standard prefabs for each template with fixed header locations and etc and then injecting text into them doesn't really make sense.
Now, I suppose as a workaround in my case, since tutorials and wikis are one-offs I could just store them right in the scene instead of using prefabs. The drawback there is that if I want to change the general appearance of my text containers I'd have to do it one-by-one instead of just changing a base prefab.
So I guess I would end up with a system where I just mock up each long text in the scene, then have a script that extracts and stores that text. Then at runtime, instantiate the prefab and inject the text. I'd also have to build an editor interface for displaying and loading each text into the mock up container (for later editing). Not the workflow I was hoping for, but once set up it should require minimal maintenance - I'll give it a try this week.
Have you considered reporting this regression to Unity? I know it's often a waste of time, but in my experience, they've actually been pretty quick about actioning bugs and regressions in Unity 6.