It doesn't happen outside of WordPress, so I'm posting here.
I'm using a textarea field in a custom form, and before I have a chance to do anything with it, I find it's escaped:
gives: Let\'s do something with this!
(BTW: No, I won't echo a POST field outside of a sandbox)
This is PHP 7.1, so there won't be magic_quotes, or safe_mode to worry about.
Tried after switching back to twentyseventeen, and with all plugins disabled.
This should be basic functionality, but it has me puzzled.