Seems like all their stuff is open source.
It's a bit too late for that. They undermined their reputation when they started permanently keeping sensitive user data in the cloud (like a list of every person you contact), and then again when they refused to update their privacy policy which lies to users about their data collection practices, and then again when they killed off the ability to get both "secure" communications and unsecured SMS, and then again when they started adding weird cryptoshit nobody asked for. Signal seems to be telling people as loudly as they can not to use/trust them.
To have self-hosted chat services, you either need a niche enough service that you'll never have two parties that would want to talk to each other while being on different servers, or federation. Signal chose the former, so here I am with eight communication apps on my phone.
Maybe the next best thing could be to support multiple servers, like how email clients let you fetch data from more than one email provider, if they're so worried about federation inhibiting their ability to control the ecosystem that they plainly won't go there and hold speeches about how harmful that situation would be. Then we could have self hosting and also Signal wouldn't have to care about federating with my self-hosted server.