My understanding is that this technology was developed only by ACINQ. I understand that some wallets are now working to include or use trampoline nodes on their backbone electrum is a good example but also muun tweeted weeks ago that they are trying to implement it.

I'm collaborating with a few people on this topic and more, hopefully released by TABC. TLDR: it is currently privacy degrading until the network adopts it, then it gets really interesting.

Is anyone using Phoenix still? I thought we had all moved to Muun for the noncustodial option.

My understanding is that trampoline routing doesn't aid in privacy but instead helps with finding routes for lightning nodes that are behind on gossip (like a mobile phone that's been offline) or lack the resources to store the network graph.