I do think acid reflux is a likely culprit, and my docs tell me it can still be happening even when you don't ever get an obvious 'vurp'.*   So I try to take my Protonix (or that sort of thing) regularly.

Whatever the reasons, my lower range finally did begin to come back in the last 6 months. It's almost fully recovered, at least when I warm up thoroughly.

But I'm noticing if I don't sing or warm up daily, I'm getting to the age where that 'elderly' sounding voice can easily creep in. Gotta rest between gigs, but also gotta warm up, even more than I used to have to do.