Googles change to debian-testing reflects my own rite of distribution passage. I used manual /etc/apt/sources.list upgrades within Ubuntu so often, after Unity was abandoned, there was no reason to stay on Canonicals flavour.
After a year on testing I went to unstable and subscribe to bugs affecting me from time to time. Usually there's either quickly a workaround or a flatpak as last resort. Not something I'd install on the family pc, but testing or unstable is a good choice for developers.
Contributing to Debian and adjacent projects like GNOME and freedesktop.org got easier (or standardized?) since they all run a gitlab instance each.
Of course, it’s not officially supported by anyone, so when the (very) occasional bug does hit, you have to sort out rolling back to something that works yourself. Sounds like Google automated this side of things?