i guess https://mattermost.uservoice.com/forums/306457-general/suggestions/12818799-off-the-record-messaging is high enough in voting 
unfortunately still “under review”. not a programmist so i won’t help in developing e2ee, sorry
btw. about “matermost should have it too” sentence… i suppose that more accurate phrase is: mattermost WILL have it too (the question is when)