It's Diablo. I'd assume everything respawns. Having said that, just curious, but if you were playing solo and assuming you weren't loot farming, why did you ever go back to boss maps to see them respawn anyway? Seems to me you could avoid spoiling your immersion by just pushing forward as you would in a normal single player RPG.
I thought there were areas that I had to cross again having bosses. It's been years so I could be remembering wrong.
I guess it's not really that big a deal since Diablo and its clones aren't my preferred type of RPG anyways. I just wish they had included an option to turn off respawning in single-player mode.