HTupolev

In the sense that 26" wheels are no longer widely used on quality new bikes designed for off-pavement use, sure.

Old-school MTB framesets do tend to be pretty heavy, and their geometry can be pretty weird for road-esque builds.

But if you're "getting smoked" compared with folks on current drop-bar gravel bikes, there's either something seriously non-optimal with the build, or the other riders are just much stronger than you. When I bring my drop-bar '84 Stumpjumper to rides with folks of similar ability on modern bikes, it's a competent performer.



If it's in good working order, provides you with the postures you need, has the gearing you need for the riding you're doing, and if the tires you've chosen are appropriate and fast, it really shouldn't be holding you back all that much.

What in particular are you having trouble finding?

High-end and/or tubeless rim-brake 26er rims are getting slim, but plenty of low-end wheels are still available new, as are a number of competent rims.

The market certainly isn't as good as it is for 29er or 650b, but the size isn't horribly obscure just yet. A few manufacturers are still spec'ing rim-brake 26er wheels on new low-end MTBs, such as the Trek 820. And there are enough old 26ers out there that tire selection is still reasonable.

I'm actually building a new wheelset for my Stumpy right now. It's nothing fancy, but my current wheelset is bottom-end and ultra-heavy, so it should save a tangible amount of weight and have much better bearings.
I'm using 36H silver Sun Rhyno Lite rims, 36H silver Deore XT T780 hubs, and Sapim Laser spokes. Hubs+rims+spokes+nipples are about 2000g for the set; it's no featherweight wheelset, but the weight isn't catastrophic either, and the parts weren't terribly expensive considering that they were bought separately. With skewers and tape and tubes and my 53mm tires, they should end up a little over 7lbs.