I don't know if other communities landed in the same situation as Arqade, but I'll quote a portion of the answer on our Meta post.

Given that it has been Jan 13 for several hours already (by UTC time), you've given Arqade's community what, 10 minutes heads up on this experiment?

At the time of writing this answer, Arqade was not listed on the Phase 3 list (and wouldn’t be for multiple hours*), and other than a passing discussion about potentially supporting ads back in July last year with the mods, no info has really been shared with us.

You read that right - SE included us without (a) at least notifying our mods about it and (b) did so before phase 3 had even been edited into the post I'm creating an answer for.

*Yes, that part was edited in later.

I don't know if other communities landed in the same situation as Arqade, but I'll quote a portion of the answer on our Meta post.

Given that it has been Jan 13 for several hours already (by UTC time), you've given Arqade's community what, 10 minutes heads up on this experiment?

At the time of writing this answer, Arqade was not listed on the Phase 3 list (and wouldn’t be for multiple hours*), and other than a passing discussion about potentially supporting ads back in July last year with the mods, no info has really been shared with us.

You read that right - SE included us without (a) meaningfully notifying our mods about it and (b) did so before phase 3 had even been edited into the post I'm creating an answer for.

*Yes, that part was edited in later.