For those tired of waiting who need a reliable tracking mechanism right now, a temporary workaround is to create an extra URI with the tag you want, then click the gear to enable the match mode dropdown menu, and set it to Never. Unlike notes and custom fields, the URI is actually indexed for searching, and setting to to never match prevents any possibility of a strange false positive autofill.
It’s not a perfect solution, and the UX certainly doesn’t stack up to a proper tag system, but for my use case it works. I absolutely needed a way to track which accounts had been secured with my Yubikeys for future reference in case a key is lost or damaged. This lets me do it without cluttering up the name portion.