CLAs aren't used by a lot of FOSS projects but the practice seems to go beyond just Canonical. I guess it means the license is easier for the organization in charge of the maintaining the software to change, but presumably if they did (and I highly doubt they would) change the licence of their products to All Rights Reserved, the old version (like with Reddit) would still be FOSS, right? Someone could then simply fork that and keep the software FOSS.
The install docs there are outdated. If you spot outdated repos then make topics in the snapcraft forums filing the issue, the snapcraft team are very keen on keeping snapd up-to-date across all distros. Also, as of snapd 2.27 (released a few months ago), the core snap can re-exec, which means that snapd can keep itself up-to-date without new distro packages needing to be released (though idk if this works on all distros).
As for adoption... I'm not sure. Thanks to the great work of snap advocates like Martin Wimpress, snaps like Spotify, Skype, JetBrains software, and Slack are hooked into upstream CI and updated by them, automatically. Some other software like Peek and MuseScore are also distributed by upstream. Though folks like Martin have helped them to get going, they are now the maintainers of their own snaps. I think it would be correct to say that with FOSS software, Flatpak has more adoption, but it is a close-run thing. Snapd seems to be stronger with non-FOSS upstreams, Flatpak seems stronger with FOSS upstreams, but there are exceptions and, for the gaps in both, usually snappy/Flatpak advocates maintain packages. Canonical themselves maintain various packages like those for LibreOffice, Chromium, and some GNOME applications (as a distro maintainer would, but this time they're all fully up-to-date like they would be on a rolling system, but without the stability questions).