@todd-weaver (Purism CEO) was interviewed on the Late Night Linux Podcast.
You can listen to it here: https://latenightlinux.com/late-night-linux-episode-57/
The summary is here: https://brimborium.net/notes-from-lnls-todd-weaver-interview/
And there is very good news for Signal on Librem 5 !
Will Signal be supported?
Community folks are working on the APIs to have Signal work. Purism have connections with signal, of all the applications out there it is the most likely one to be included by default and meet the criteria Purism have.