Compare to turbo/stimulus from Rails world; I would put the approach in the same basic category, that being: Custom HTML attributes triggering standardized behaviors, with HTML sent over the wire for partial page updates.

I wonder what other platforms are going on right now in this same category -- anyone have more examples?

And in general how much "mind share" (or really development-share) it's getting.

I'm not super familiar with this but what I can say is that many of these things were inspired by Phoenix LiveView, and as far as I know LiveView is still the one that works best.

Here's a list of LiveView-inspired projects (including StimulusReflex): https://github.com/dbohdan/liveviews