I understand this problem and want Discourse to reach as many people as possible. That said we also need a modern experience and we need to take a stance around the minimal requirements for a modern experience.
I feel like the long term possible solution here may be a “html+css” writable view.
All your 800 users will still be able to read and interact via email, that is not going to be lost.
Maybe the long term change here is that we will eventually provide a minimal ability to login and write in our html view so some of the more unfortunate can still interact on very old devices.
It is important to note that bargain oppo and techno phones will still run Discourse after this change, we are talking here about hand me down devices generally or people who can not afford to upgrade for 10+ years.