Did they remove the "Hair BSDF" "Principled Hair BSDF" shaders from Blender in versions 2.90 and 2.91 Alpha?
2.90:
2.91 Alpha:
compared to 2.83.0:
It's likely because you're using Eevee - it used to show up in both render engines and just not work in Eevee (renders black), but now it looks like they've removed it from the listing since it's not supported unless you're using Cycles.
Thanks, @jlampel, this was the solution:
And my heart is beating again 😉.
