You can do that already, in a way. Twitter lists provide a way to 'subscribe' to someone's tweets without actually following them and putting them in your main feed / letting them DM you. I love @SwiftOnSecurity, but she tweets too often for me to follow her, so I have her on a separate InfoSec list, together with @TroyHunt and others.

If there's someone you feel socially obligated to follow but find annoying (or they decide to go on a rant one day), you can Mute them and you'll still be following them without seeing their Tweets.

I find Twitter only works if you can be ruthless in who you unfollow. As soon as you worry that unfollowing someone will offend them, it starts to fall apart.