With regards to remastering older albums so that they meet the criteria for Apple Digital Masters - this is up to the artists and the labels. They need to remaster the albums so that they meet the criteria set forth by Apple to be an Apple Digital Master (formerly Mastered for iTunes). From 2012-2016 a lot of artists did this, but the pace has slowed for sure. The purpose of ADM is to create the highest quality masters possible for the lossy format, with improved dynamics, no clipping, etc. The fact that an album like Rumours by Fleetwood Mac that has been remastered for Hi-Def formats but is not considered an Apple Digital Master leads me to believe it did not meet the criteria set forth by Apple, while their other remasters did.
As far as iTunes Match, sometimes they are, and sometimes they aren't the ADM version. The best way to find out is to "Remove download" and then download the version from the cloud. If the file shows the ADM badge, then you have the ADM version. If not, then no. I have found some artists typically revert to this automatically (Bruce Springsteen, Billy Joel, Bob Dylan) while other don't. It is up to the artists and their labels - they effectively control the content that is in the Apple Store.
As far as Apple Music files and ADM, it makes no sense to me why downloaded files that are Apple Digital Masters are not identified as such. Seems like a wasted opportunity. Particularly older albums where Apple may carry multiple copies of the album. Identifying Apple Digital Masters in Apple Music, as well as in the downloaded Apple Music files just makes sense and seems like it should be easy to do.