Sorry, I mistakenly assumed you were talking about disk storage - sure, if you’re designing your own solution, definitely use tags! Although the ones Gmail uses aren’t really portable in my experience, so you’re forced to use their mail client. That, however, is pretty much unavoidable if you’re putting a new spin on established protocols like they’re doing - maybe those changes will get picked up by other clients, maybe they won’t, who knows?

That’s true, but since we’re stuck with the file/folder system for all intents and purposes, you should be able to replicate that behaviour by making those tags part of the filenames (like rent_lease_landlordX.pdf) and searching for (parts of) filenames instead. But yes, a dedicated system would of course be preferable.

Why not just use soft links instead?

That’s something I would disagree with though. “Sticking with plain HTML and CSS” is way more work, and often has significantly less functionality, than building a website with a framework.