So in short, this is the thread (and associated poll) for discussing whether to remove the option to turn of z-level functionality, and make it mandatory like DDA did several years ago.

Note: I am NOT talking about the 3D vision functionality! That is its own can of worms (and other assorted bugs) that are far as I can tell dependent on baseline z-level behavior in the first place, so it wouldn't even be on the chopping block until we're well past having locked this mechanic into being mandatory. For clarity, it's this setting:

3D vision (the famously buggy one we don't want to make mandatory anytime soon) by contrast is the one in debug settings instead of world settings.

The following content are all affected by whether z-levels is on or not:

1. Helicopters in general it seem can't fly at all without z-levels turned on. Rather amusing that heli functionality was added to DDA well before they made z-levels mandatory, shows what a red-headed stepchild it was during development.
2. Last Flight scenario indirectly depends on z-levels being on to make any sense at all, given it focuses on being able to pilot a helicopter.
3. Climbing up stepladders and drain pipes does not work with z-levels turned off. Stepladders are especially jank, their item form prints an error message when activated saying ladders can't be used with z-levels off.
4. Elevated bridges, and thus the ability to travel along a river properly by boat, is locked into being a mod duee to z-levels being optional.
5. Z-level ramps are oddly semi-modlocked due to this as well, and the ability to construct them is shifted to elevated bridges as well. I wasn't aware of this when I added the bastion fort, making them currently the only other place where ramps spawn and they'll be non-functional with z-levels off, not even acting as stairs.
6. The lack of ramps in pure vanilla hinders their potential use in LMOE shelters (being covered in fake slopes that presently work like stairs), parking garages (imagine being able to drive out of one using ramps), someday even hills and/or mountains.
7. The ability to see the surrounding terrain when on a higher evelation is, predictably, locked being z-leveled being on.
8. \ hauling items up and down stairs is locked behind z-levels being on.

Main counterarguments, things which are negatively affected by z-levels being on:

1. In theory, the main one is performance.
2. Explosive cratering behaves inconsistently depending on z-level mode. Namely, holes only seem to get punched down to lower levels when z-levels are turned on. Though to some that might be a good thing given how janky z-level craters can be.
3. Monster handling of stairs is famously kinda dogshit when z-levels are on, as the game trusts their pathing to handle the existential crisis that is a target on a different z-level, while with z-levels off there's some ancient magical code chicanery for automatically shuffling zeds up and down stairs they're nearby when you use them to flee.

My general takeaway from this is: Many systems in the game seem to have been designed around the assumption of z-levels being on as the norm that only people playing on potatoes even worse than my laptop change from the default, and a lot of core mechanics basically flat-out stop working when z-levels are turned off. Nothing looming in the future really makes z-levels any more mandatory than they already are (we can only just add yet another mechanic that does nothing when z-levels are off, after all), but a few existing things would become less hassle to work with if we did what DDA did already.

In exchange, we mainly get a performance drop that should most likely pale in comparison to basically anything NPC-related, and a couple janky but minor mechanics become mandatory to deal with (one replacing a very old and probably fragile legacy mechanic, at that).