I have always taken it as gospel truth that deep-cycling a lead-acid battery was one of the many Really Bad Things (tm) you could do to it.

Ditto, at least WRT repeated deep discharge of a non-deep-cycle starting battery. If trying to improve battery performance by removing sulfation, then the routine approach I'm familiar with is governed "equalization" overcharging. Some techniques I see recommended include discharge, but not below 50 percent. So-called "smart" chargers and regulators include equalization among their phases, but not the typical automotive alternator/regulator setup AFAIK.

I have 4 big batteries under my protection, so I stand ready to learn new tricks!