Can I install a disc-brake pannier rack on a bike with V-brakes? I have a MTB with V-brakes. 14-15 years old. I want to get a pannier rack. Will a rack suitable for a bike with disc brakes (eg Topeak Super Tourist DX) fit a bike without disc brakes? I’m not sure if I’m going to buy a new bike in the next year which would have disc brakes, so am trying to avoid buying a second rack at a later date.
Disc rack isn't an exact term. It means the rack will probably be a bit wider and make some effort to clear a disc fitted to the rear hub.
Disc model arms are brought on so the rack supports will be less likely to foul a disc :

Vs.

They both are essentially the same thing and the main thing is that new and old frame has the mounting points shown in the image:

If it lacks these then a rack will be more difficult to install. Some new bike frames will not be built to take racks. So buying a disc type rack doesn't guarantee compatibility. However it's more likely to fit.