As said, priority boarding is not an official *S benefit, in line with other bottom-tier elites like OW Ruby. However since UA sells the Premier Travel product, they are pretty much the only airline to have *S boarding. On busy flights on UA I usually hear a "2P/CO*S/Premier Travel/*S" boarding call after the usual F/GS/1K/Pres etc. If the plane is going out half empty, they sometimes just call all the *G and above on at once.

I've never seen priority *S boarding on LH, LX, AC, US, SQ, CA, OZ or NH. However, I'm usually on the plane at *G boarding so I wouldn't know for sure what happens outside. I think TK might have priority *S boarding.