I thought the LocalOnlyHotspot route was the way to, but as @edsappfactory.com said in the comments – it only gives closed network, no internet access.
In Oreo hot-spotting/tethering moved to ConnectionManager, and its annotated @SystemApi, so (nominally) inaccessible.
As part of something else I was doing, I made an app and put it on github here. It uses reflection to get at the function and DexMaker to generate a subclass of ConnectionManager.OnStartTetheringCallback (which is also inaccessible).
Think it all works okay – bit rough around the edges, so please feel free to make better!
Relevant bits of code are in:
I lost patience trying to get my DexMaker-generated callback to fire the MyOnStartTetheringCallback so all that code is in disarray and commented out.