Everyone who does a self assessment has now seen their first section 24 tax bill. There'll be some who had no idea about the changes, thought it wouldn't apply to them or didn't realise he bad it would be. Some of them will now be late payers because they didn't have the money on hand to pay it. I expect a fair number of BTL properties with motivated sellers bto hit Rightmove in the next week or two.
Then you've got the landlords who don't do a self assessment. HMRC hasn't been bothering to chase a lot of these since the tax owed isn't worth it. A lot of these are now worth chasing and will soon be receiving pants soiling letters.