It says my rank is deteriorating even though solo queue isn't out yet...

It's just a false notification. Decay is indeed disabled right now.

Wait what? Solo queue has been back for like a week or something. What do you mean it isnt out yet

Wont matter. Your MMR is unaffected by decay and your MMR is going to be reset in 2 months anways. The reset will affect you the same regardless of whether you play a match or two to prevent decay or you let the account decay all they way to gold 1. Either way its going to use the last value your DQ MMR had, add 1200 to that, and then divide the sum by 2. Since there is no more DQ and there is no solo queue at the moment your last DQ MMR value should be static, unchanging, until the reset happens anyways.

Its a visual bug, you can't decay in preseason.