Turns out you’re right. I just uninstalled the CDM and YouTube indeed works!
I’m still absolutely, positively sure that m.youtube.com started gating it for me last August without a doubt. [0]
Maybe they pulled some temporary A/B experiment on me? I’ll probably never know. Thanks for the correction.
It's m.youtube.com that seems to require the DRM thing, at least for me. Have you tried that?