I think a better case can be made for it in modules and setting materials than for rules.
Regarding obsidian.md, I have never heard of that and searching for their website doesn't really tell me anything about how it relates to an SRD or game material in general.
Obsidian.md is simply a note taking/writing app that's note solely just for RPGs. But growing TTRPG community has adopted Obsidian.md for RPGs. It's being used for everything from Campaign Management to running live sessions. It has plenty of plugins for RPG content like a fantasy calendar, statblocks that work with an initiative encounter, a graphical Dice roller (I've got mine set up for open-ended rolls on percentile dice), and so much more.
The only issue with it is that it takes a lot to put all your own stuff into the program. The RPGs that get the most attention are the ones that have an SRD and the people behind the TTRPG OBsidian community have already loaded quite a few SRD's into Obsidian.
Unfortunately neither HARP or RMU have an SRD so you would have to enter in all that info manually. Lucky for me I already had a lot of HARP programmed into Realm Works and someone created an importer plugin that will convert Realm Works files into markdown files so it was a much easier transfer than it could have been. It is an incredible program.
Here is a great beginner video by Joshua Plunket, a patreon I support.
https://www.youtube.com/watch?v=qV3Dp1Kki1E&ab_channel=JoshPlunkett