I’m the author of org.gnome.DejaDupDevel from that list - and I don’t like the .Devel approach.
If memory serves, that recommendation of .Devel means that the two apps aren’t two separate namespaces, but rather some tools (like flatpak) treat .Devel as a sub-namespace of the normal app. I.e. the org.gnome.DejaDup flatpak app can talk to org.gnome.DejaDup.Devel over dbus without extra permissions and claim dbus names there, etc.
Not that it would intentionally, but it just feels like the wrong way to approach the problem - conceptually to me, I’d want to treat the two apps (stable and nightly) as entirely separate and distinct.
So for my part, the org.gnome.DejaDupDevel name is an intentional choice. Is there a particular advantage to the .Devel approach?
I think it’s cleaner to have app names separated from the Devel string using a '.' delimiter than appending to it.
Also, flatpak app and it’s nightly version are developed by the same developer / team, so I guess sub-namespace issue will never be encountered (though I understand your concern) ?