What is it that you want to achieve?
Over the decades the only true constants that will guarantee you future proofing and system interoperability is plain text files and their file names. The moment you start thinking about adding metadata outside of these 2 things, you start getting “locked in”, regardless the operating system or app you are in. Even
xattrs are not future proof as filesystems evolve over time, ditto to many plugins in Obsidian.
One other constant is perhaps the directory or file structure, so a nested structure will give you another “dimension” of portable metadata.
If you use graphics, sound or videos in your files, that already is brittle in the context of interop and future proofing.
If you really want to make sure that files you create now are usable 100 years later on some yet-to-be-invented system then I would suggest using
vim (or other such plain text editors) and work with plain text files with none-to-minimal markups eschewing any form of graphics (except ASCII art), sound, video etc. To make presentations, do it the way we used to when all we had were text terminals – use plain text files, and count lines to separate out “slides”.
I would also not shirk from using XML or JSON (or CSVs for tables) since they are nothing more than work-arounds to increase the dimensions of flat text files with nothing more than text, and you or your successors should be able to create scripts to manipulate them where ever they end up.
This is not some slight on your ideas, it is something that I have been thinking about as well having used everything from VAX to unix to NeXT to BeOS (remember that?) to Windows to OSX etc, etc. and numerous filesystems. It really comes down to what you want to achieve, which will in turn inform you on what compromises you need to manage.
I am a heavy user of DT3 but I know that if I am going to switch away from the Apple ecosystem one day, I will easily be able to extract my data but I’ll have to discard some metadata that Devonthink3 provides for convenience such as ratings, colour labels and Devonthink3 tags. It’s why I make minimal use of them except when I need to do something efficiently now and those tools allow me to quickly, with minimal effort, manipulate my files.
I also place important metadata in the filename. All files have a date or two in its name since even creation- or modified datestamps are not sacrosanct, and sometimes context (e.g. project name at the end). I make full use of the 255 bytes.
My philosophy is to make use of the best tools available now but keep the underlying data that I want to preserve as simple as possible, knowing that a majority of the data I receive or generate does not need to be preserved (since I’m not someone future generations will want to study!).