Thank you for your suggestion. Currently we have no plans of adding this functionality (neither for other platforms like Steam, Twitch, etc). But we always welcome contribution, and if you feel up to it, we are more than willing to look at any Pull Request that might enable this.

I am working on taking this on. It should be ready eventually.

@bennyman123abc any updates lol

Wow, I completely forgot this issue existed. I supposed I could see if I did anything with this when I get home. Ping me again Tuesday at 10:30 AM CST

I did some work on this a good while ago, based on the idea that a rich presence API might involve code that can't be linked into OpenTTD itself due to licensing, so it requires an external dynamic library to be installed.
OpenTTD branch: https://github.com/nielsmh/OpenTTD/tree/rich-presence
Library for Discord support: https://github.com/nielsmh/ottd-discord

Thank you for your contributions, Niel! I will do a bit of research on including the SDK DLL in a project, and I can contact the Discord legal team if I need to.

I will get back to you and let you know if we need to bundle the repositories separately, or if we can include the DLL in the game without issue.

Thank you!

One important point that isn't solved with code is that to implement the Discord API in the game, OpenTTD needs to be registered as an organization with Discord to get API keys, and someone within the project needs to be owner(s) in that org. Without that it isn't becoming an official feature either.