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Chromium based browsers treat localhost as a secure context, so HTTPS isn't required to access APIs like the clipboard, web serial, etc.

Not just localhost, but *.localhost too. It's not just Chromium, it's a standard that Firefox supports too (Safari as usual is kind of spotty): https://www.w3.org/TR/secure-contexts/#is-origin-trustworthy

Is that a recent development? It was not so a couple years ago, at least not when a non-80 port is specified.