The instance I'm using (programming.dev) offers five custom UIs and they are mostly available under subdomains by the first letter of the UI name:
Tesseract (t.programming.dev)
Photon (p.programming.dev)
Alexandrite (a.programming.dev)
Mlmym (old.programming.dev, because this one tries to mimic old Reddit UI)
Voyager (v.programming.dev)
Makes sense to use a lower level domain for that, I think.