Currently running 4.2 and the following script doesn’t work.
/ip proxy access set [find dst-port=80] action=deny
This is used as a scheduled script to turn off internet access for my kids after a certain hour. 
:foreach RULE in=[/ip proxy access find dst-port="80"] do={ /ip proxy access set $RULE action=deny }
Or alternatively
/ip proxy access set [find dst-port="80"] action=deny
But I like a loop better since it indicates very clearly that it works on multiple elements.
Hmm.. Why would something like that change between versions, especially for a number? I was using this in 3.x and it worked fine without quotes around the 80. Then again, they changed /ip web-proxy to just /ip proxy, which messed things up too..
Anyway, that worked fine. Thanks.
Precisely why. You answered your own question
I agree with the loop method fewi mentioned as well. I’ve been noting it on everything I do involving find. It’s worked for me from v2.x → v3.x → v4.x with very good consistency. When you combine [:tonum], [:tostr], [:toip], and [:tobool], you get about 99.9999 % consistent between any version of RouterOS.