In Cloudron 6 there is Full Text Search (Solr), in the docs it says "Both, Roundcube and SOGo can take advantage of indexed search.", does that mean that IMAP doesn't take advantage of it?
Solr is powering the IMAP server side search feature, so whatever client uses that will benefit from solr.
Some clients like Thunderbird, which are set to cache all emails offline, would not use it but would search locally.
I found that in Thundberbird, if you press Ctrl+K, it will use IMAP search.
On Android K-9, there is Settings -> Account Settings -> Search -> Enable server search.