Thanks for commenting on this. core was the default base before core18 and core20 were developed. As a result, many snaps were originally built using core and may still be using it. The tree snap is a good example as it hasn’t been updated since 2019.
core is based on Ubuntu 16.04 LTS which is now supported only via Extended Support Maintenance (see Snapcraft and Extended Security Maintenance). Consequently, core was dropped from the main development branch of Snapcraft as Snapcraft’s architecture and plugin system changed.
Snapcraft 4.x remains compatible with core and will be updated and maintained on its own track for the duration of the the ESM period of Ubuntu 16.04 LTS.
I’ve attempted to update the document to make this clearer.