Reddit is removing the Direct Message feature, and all active conversations will be archived so you won’t be able to continue ongoing conversations. Reddit also deleted all bad reviews for their app awhile ago. Its rating went from 3.4 to 4.5 stars overnight.
Also DMs work on both new & old Reddit. Chat only works on new Reddit.
(Unless something has changed recently.)
Oh, and some open source app you build by yourself and put your API key in only work with DM because those app is no longer maintained.