I accidentally selected all messages and hit the mute icon, and all incoming messages were muted. Before I selected all messages again and unmuted, I deleted a handful of messages that were still muted. Later new messages came in from the people whose messages I had muted/deleted and they were still muted. Consequently I missed their messages until I opened Textra for other unmuted messages.

I've not found a way to locate people I've muted so I can unmute them other than to wait for a message from them. An option to show all contacts and provide a means to mute and unmute individuals would be really helpful.