I set up Whatsapp and Telegram bridges with Tuwunel today. The config file for mautrix-whatsapp indicates Dendrite should work as well so it seems just about every server implementation supports bridges?
Former Dendrite user: Yes and no. Mautrix stuff does work with Dendrite, technically, but only partially. For Mautrix-Discord, I was able to get DMs mostly working, but only DMs: bridging servers just never really worked at all. I'm honestly not 100% sure why this is the case, but it's not worth worrying about Dendrite anymore. It is a dead-end server that never worked very well. Up until shortly before it was discontinued, none of the Mautrix bridges worked well enough to actually use.
This is not true, at least today. Continuwuity, which is an alternative server implementation, and its predecessors support bridges very well.