1. 5

This is a discussion from 2014, it is interesting to read in retrospect. Intents are implemented in the XDG desktop files but don’t seem to be widely used. The FOSDEM talk mentioned can be found here. For the selection of default apps, there are specific distro solutions such as Debian alternative system and Xfce Preferred applications.
Meanwhile, intents as dbus interfaces have indirectly taken form via desktop portals.