https://github.com/signalapp/Signal-Server/commit/95f0ce1816...
Commit message "Support for advertising payment addresses on profile". It's likely related to the current public announcement that Signal added payment support. Back then the feature was likely less fleshed out, and only later they dropped the idea of integrating ads. I guess they didn't release the code so that they could make a proper announcement, once it's ready, which happened today, so they also released the code today.
I don't think they should have done this though. Instead they should have privately forked the server, made a private test network, and used that to test the feature. The official public server could have kept running the latest master and its source could remain public. They did use custom clients to test the feature, at least I couldn't find any commit in the android client whose description mentioned payments before commit c42023855b1eb8abb669da1856f8377457a0edf1 "Service support for Payments".