At least BRP (the system behind Runequest) and d6 are actively developed and there are games, new and old, using them. They are also rather popular, at least if one browses rpg.net.

T&T I don't know. I think there is a new edition coming along?

This is true. Though I will say that modern iterations of Runequest and d6 (namely OpenD6 and its derivatives) resemble the original systems only barely. Same is true for T&T, which after its 5th Edition became likewise unrecognizable.

My point isn't necessarily that there shouldn't be modern versions of these games—obviously, D&D is the most popular TTRPG (at least in the US), and itself barely resembles the three brown booklets of '74. It's that by going back and discovering what made the originals so great, we can carry these lessons forward and make loving modern interpretations like Old School Essentials/Dolmenwood.