I’ve never used Mercurial, but a simple one based on the explanations and my experience with Git:

Locating the branch a commit originated from. If a git branch has been merged into (or rebased on) main or another branch, there’s no way to tell which commit came from which branch. But sometimes I’d really like that information to figure out what prompted a certain change. Without it, I need to use external tools like a ticketing system and hope the other developers added in the necessary information.

Depending on the stuffing, I might actually rather take the seat, just because it’s got armrests.

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.