Side scrolling games...

There were a few back in the NES era but it's hard to recall them all. One I can remember off-hand is River City Ransom, though that was more of a brawler. Even that would respawn enemies if you returned to a previous screen but that was because there would be no game otherwise. I'm sure there are better examples.

It wasn't necessarily a technical limitation. If a programmer back then wanted to, they could keep a counter to see how far you had progressed rightwards on the map and if you went back to the left, they could run a check to see if an enemy should spawn. That is to say, if the map is 200 tiles long and the player starts at tile 0 and works towards tile 200, and you have enemies spawn on tiles 10, 20, 35, 50, and 75, then if the player gets to tile 100 and turns around, any spawn point on a tile with a value less than 100 would not activate. (This assumes your bog-standard side-scroller with not much verticality or backtracking.)

However, it wasn't really deemed important. You were supposed to be moving forward and if the enemies respawned because you backtracked, well, that's on you, chum. There's no sense for extra code and memory use for something so insignificant. I suppose it was also just considered the norm at the time. Video games were still new and it wasn't really something people complained about or that felt like a bug.

Many games would even incorporate it into gameplay to ease frustration. A good, semi-early example of this is Super Mario Bros 3. Early in the first stage, there's a ? box on the ground which contains a leaf (a valuable power-up) and to get it, you need to stomp a Koopa and toss his shell at it. If you somehow missed the shot, you could run ahead a bit and come back to respawn the Koopa to get another chance. Others would make enemies drop power-ups which allowed you the opportunity to farm.

Games like Castlevania and Ninja Gaiden, on the other hand, could get downright obnoxious about it. They were the Donny Don'ts of enemy respawning. There was no gameplay advantage to the respawns and they had such relentless enemy placement and patterns that even a few steps back could screw you.