I recently reported this to support and asked for the same, I was told to post an idea here but you already did :D

I cannot use this service for our legitimate email servers without floating IP + reverse DNS. From the response given here https://www.digitalocean.com/community/questions/how-do-i-set-the-ptr-for-a-floating-ip it seems that you are not allowing reverse DNS to block spammers, but in fact you are blocking legitimate businesses. The right thing to do would be to find another way to block spammers.

I have been fighting spammers for 15 years. Cloud computing was a godsend for them. The latest tactic is to spin up a few servers, send out a bunch of spam on fresh IPs, then make them disappear. I’ve been fighting one spammer for 10 months who has used over 300 ranges of IPs and counting. This tactic you have of not allowing reverse DNS for floating IPs is not stopping a spammer that uses IPs for one day. It is stopping legitimate businesses that want to establish a long term reputation of credibility on a certain IP, something that spammers do not want to do. You are hurting and not helping.

BTW ... AWS has had this feature since 2010.