S1RK4M is correct. Here is the details:
Developer comments: The “Prefer”/“Avoid” player system was designed with the best intentions; however, it’s not currently performing in a way that we feel is healthy for the game. While the “Prefer This Player” option is more or less working as intended (and is thus still enabled), the “Avoid This Player” option has impacted the matchmaker in negative way and led to some very poor player experiences across the board. Although we like the idea of being able to say “hey, I’d prefer not to play with this person,” the implementation of the mechanic is not where it needs to be. We’re still looking at ways to iterate and improve upon features that empower players to reduce toxicity and harassment in their games; we just want to be sure these features don’t accidentally punish positive community members as a side-effect.
Additional questions? I have a handy FAQ here that might cover them.