I've set my preference for "Pressing Return in Mailbox Search Field" to "Jumps to Mailbox". It's a feature I've really benefitted from in the past. But once I upgraded to Catalina awhile ago, it stopped working. Now when I press Return in the mailbox search field, nothing happens. I can still click on the arrow to jump to the mailbox, but I'd rather have the Return key work instead.
Doesn't work for me either since Catalina. I just assumed the feature is gone, and that there is zero point in reaching out to the long-absent developer.