For rspamd being able to learn ham or spam from messages being moved into spam/junk folders or out of them dovecot needs to know how spam/junk folders and trash folders are named.
The former rules narrowed the folders being recognized as spam/trash down to just 'Junk, SPAM, Trash' (case-senistive).
Since users and admins can change the foldernames and write their own seive filters to use those folders I think it is a big improvement if more folders will be recognized.
The change is supposed to accept some more commonly used folder names for spam and trash in a case-insensitive manner.
This is reasonably important for the performance of clients such as Delta Chat. The plugins are bundled with dovecot by default (see https://wiki2.dovecot.org/Plugins ) so this should not be disruptive.
