They may be doing this in some countries and not others or just A/B testing users based on some random allocation of accounts.
I would not be surprised if Google wants to understand how effective something like this is in terms of getting people to disable adblock vs them just being like "welp fuck YouTube I guess." So would make a lot of sense to A/B test this rather than roll it out across the board.
Yeah, I subsequently learned it may be rolling out in different countries. I meant to update my comment as such but forgot :/ FYI: In case anyone's interested, I'm in New Zealand.