The list of software components that the foundation maintains is on the Maintainers page.
I'm hestitant about adding it here, as I think "support" in the context of the Compatibility page means something else than the sense of "software that is supported", and rather means what the supported software in turn supports externally (e.g. given that the Vector skin is maintained/supported, what browsers and PHP versions does it in turn support?). However a pointer from here to Maintainers may be helpful to have a path for when others may be looking for the same thing here.
As a general rule of thumb, I would say all software that is in production is and must be supported and maintained. However, it is not owned or stewarded by a specific team at WMF, and indeed not always in active development with new capabilities. MonoBook can be described as being in maintenance-only mode. However I would expect it to accomodate for changes that happen in browsers and other parts of MW to remain compatible, just as we do for any other skin.
For some software the lead maintainer can indeed be a group or individual outside the foundation, as is the case with Wikibase/Wikidata, and Timeless.