Tron 2.0 and some other games by the same studio (No one lives forever?) had "reinforcement buttons" that roughly doubled a monster spawn when one of the first group managed to access the button. Still a far cry from the "direct reinforcements to the rear of the player" option that would end any playthrough quite fast, but a very satisfactory repeated sub-goal.

Goldeneye on N64 had the alarms which would, IIRC, at least accelerate the onset of enemy attacks but I think also increase numbers of attackers.

Project IGI had local and global alarm systems. E.g. with two bases on the map, alerting the first base would spawn a massive number of soldiers (a regenerating pool of 4-~15 soldiers) and some other stuff, e.g. tanks, while increasing patrols in or around the second base or turn NPCs from an idle pose to a searching pose.

Assassin's Creed Odyssey (and perhaps others in the series) has a brazier on top of each fort that occupants try to reach and light to signal for reinforcements if you're spotted.

It encourages stealth, or at least stealthily reaching the brazier first to set a trap.

Far Cry had this.

In every Far Cry game, by about 30% of the game I would eventually have enough weapons go take out an entire base without a blimp.

So I would go stealthy - in a few more rounds I would learn how to headshot every enemy at the base.

I would then consider it a loss if I would not one-shot everyone at the base.

Then, I would consider it a loss if I could not knife everyone without a warning.

I have to admit I don’t get why people don’t make games with exponential skill required.

But I still can’t finish Super Meat Boy’s boss having finished every other level at perfect time and score, so what exactly am I talking about?