I see Snapcraft docs already cites core20 as being available, but I can’t seem to find it listed in snap search core20 (although it shows in snap info core20). Also, snapcraft extensions doesn’t list any compatible with it.
Regarding the base snap specifically, core20 is considered stable at this point and you can safely build snaps against it. Ubuntu Core 20, which is based off of core20, has recently entered a beta stage and so as such we expect it to not change, but we may encounter bugs with Ubuntu Core 20 that require small changes to the core20 snap, but none of these will be breaking changes for snaps built off of core20.
Extensions require migration and those are yet not available as the teams need to create their content/platform snaps based out of the recently released core20 base.
Would be great if they could release these new versions soon. Snapping an app that uses GNOME extensions and releasing it under core18 “feels old”, since Ubuntu 20.04 is out. Guess i’ll have to wait 'till new extensions is done - or fallback to desktop-helpers, right? Is that the way (not an expert snapper, for sure) ?