t2bot.io uses webhook/bot based bridging which at least doesn't trigger the "no custom clients" clause (as you aren't using any real user account tokens)
The broader question of whether bridges are allowed seems to be broadly yes as there are many other bridges out there for Discord (IRC/slack ones) and those haven't been shut down either.
I think so long as you aren't abusive, Discord don't care.
So I imagine there might be the possibility of it being allowed for Beeper as well.