Just for posterity, you asked for:

> A feature that'd let me browse by "topic" (e.g systems programming) regardless of language would be welcome as well.

Topics are exactly what you initially described. Perhaps you didn't do a very good job describing what you'd like.

Also, they're called topics. Not tags. It's in the documentation.

> Such feature would require Github to do the appropriate groundwork to become great at technology-discovery, namely

They did a bunch of work to automatically add topics for many repositories to solve for this: https://githubengineering.com/topics/