Originally Posted by
50PlusCycling
Upgrading an old MTB is getting a little more difficult. I was recently looking for a new set of wheels with more modern hubs to upgrade to a 12 speed driveline, and found that 26” options are getting hard to find. 650B (27.5) used to hard to find even at large bike shops, and now you can get them at Walmart.
Not surprising; 12-sp arrived over a decade after 26" had been superceded by 29/27.5. Doubtful that anyone developed a wheelset that's backwards-compatible with 15+ year-old bikes. (10-is pretty easily done, though)
MTBs went through a huge seismic shift in design and development, particularly 2005-2015, from 26ers to the "big wheel" era; far more than road bikes have (up to the disk/Di-2 shift) there's very little left from the classic bikes of the NORBA era.
WRT the OP's question on tires; there's still lots of legacy support; Schwalbe, Conti, Maxxis and Panaracer still have substantial lineups of "proper" 26 MTB tires (Panaracer even still makes the Smoke/Dart if you want to ride old -school) You just won't find them in an average LBS, online/boutiques are the way to go.