Synapse Software’s 1983 game The Pharaoh’s Curse is one of the earliest examples (as far as I’m aware of) of an exploration-focused side-scrolling game. Published for the Atari 8-bit family, the C64 and the VIC-20, it also got a free conversion for Amiga in 1990 by the scene group Bignonia.
It actually has a few elements that make a true Metroidvania: a seamless, highly interconnected 2d-environment made up of various screens and gameplay that forces you to backtrack through all those screens to get the various treasures and keys. It lacks any sort of powers ups nor character progression via items; all you can do to open new areas is collect keys and open doors.
The rest is just exploring all the screens, memorizing how they are connected and using that knowledge to get to the various treasures. But unlike in most Metroidvania’s, there are no slow moments, no reprieve from the platforming action. Enemies regenerate at a relentless pace, a bird is always trying to catch you and drop you somewhere else and deadly traps are bountiful on the ground.
The Pharaoh’s Curse is sort of a platformer, but more like an ancient precursor than any modern variant. The controls are imprecise, the jumping looks less like jumping and more like sliding over the ground or gaps and shooting is only possible when moving. It plays like a platformer from the time when they were still experimenting with basic gameplay elements (the perfect jump height, etc.) and had no clue what worked or not. If you fall down for example, you can’t control your momentum. This is not hard by design, but by the lack of it. Dying because of imprecise controls, because enemies generate right on top of you is the norm, not the exception.
In the end it’s neither a Metroidvania (a distant forerunner at best) nor a good game. Might have been fun in 1983 with not much else around, but while I can still enjoy Super Mario Bros. these days (it came out only two years later), The Pharaoh’s Curse is hardly enjoyable. With better platforming mechanics and better controls, this might have been fun, but as it is, it’s not.
Another thing, which at least is true for the C64-version, to win you have to play through the game 3 times. Each time you have to collect more treasures than on the previous run. Which is easy with save stats on the first one, but the last one is hard even with them and feels nigh impossible to do without. The password for difficulty 2 is “Sphinx” and for difficulty 3 “Raider”.