Assumption about the shape of your projection tend to creep into the weirdest places in complex renderers like the one in UE over time. I am willing to bet that a lot of the screen space lighting and post processing shaders had assumptions like that baked in. This can make adding a different projection type a heavy lift, even though nothing about it is magic.

I suspect they are seeing a lot more penetration into the Product Vis/Architect/CAD markets now that polygon optimization to just get realtime isn't as much of a concern.

I agree, that does seem to be the reason behind it.

It is still the kind of basic thing that everyone who has written their own renderer has done, compared to all their other ground breaking new tech. Which is funny to me.

It is for us, Civil Engineers and many other non-cs Engineering Fields.

also it's useful for isometric like games

I used UE4 professionally a few years back, and at the time ortho cameras were effectively unusable. Landscape actors were rendered at the worst LOD, for example, with no workarounds. Glad they're finally bringing them up to parity.

Anything that involves raytracing would need to change from having a finite camera position in world space, to having an infinite one with parallel rays. There are some ways to make the math work out the same, but in general... nope.

Like, consider that you never actually need to scale anything in a perspective view, because you literally can't tell the difference (all the rays remain the same). In an orthographic view, scaling is the only way to actually zoom in/out. Oops, any code that assumed your view is unit-scale will break.

Perspective vs orthographic literally affects every single one of those fancy features you like.