Can white bread be used to temporarily plug pipes for repair?
Yes, us old-timers have used this trick for decades. My father showed it to me in the 1980s. I did just a few months ago when swapping a water heater. The idea is that you plug the pipe with a gob of bread, do your repair as the bread blocks and absorbs the trickle of water, and later it softens and dissolves, flowing down the pipe.  You want to use just the inner part of plain white bread--no crust, seeds, etc. that can cause clogs elsewhere.
Just take into consideration where the bread muck will end up. If you're upstream of delicate equipment, such as a reverse osmosis system, it may not be a good idea. If you can flush it out fully later, have at it.
I'll add that I consider this a last-resort strategy. It can be difficult to make it work, especially with vertical pipes. Try fully venting the system and using a wet-dry vac first.