There is a workaround, have two libraries. What I do is have one library on MacBook air, which I mostly just stream matched songs. From this one, I put new tracks that then are accessible on my iPad and iPhone. A larger library on my Mac mini that includes all the tracks I've matched plus other mp3's. I do have a third of my lossless for listening to through my stereo. But this should also work with two libraries on different User login's on your iMac. On the smaller library, you can then erase the "matched" content you don't want stored on icloud music . Its needlessly tedious even with and when of if Apple raises the limit to 100,000 songs there are still songs in my iTunes library I don't want synced to other devices. Since iTunes Match was announced user have ask why there isn't a per track opt of match ability but apple has never answered that I know of. It's just a huge problem with the system. I think the 100,000 track limit is just too many to handle on a mobile device 25,000 requires more searching than I desire without a physical keyboard. One thing is I've haven't done this since Itunes 12 So I don't know if tracks in a library but no in icloud music works or how its listed. I haven't connected my large library to icloud music since apple music/ iTunes 12 change over. I do have my smaller library, which is on my MacBook air connected to iTunes icloud music but I've had a huge dupication of matched track problem.
Remember to back-up you files