[Blackthorn]

It's tough to tell without specific names, but I imagine a lot of particularly old* VSTs were written to use naive sawtooths rather than perfect band-limited ones, which would have terrible aliasing at 44.1 khz. Oversampling those would help a lot!

* Some people are still making this mistake, despite information on the (many) ways to do it the right way being widely and freely available!

[evo]

I wonder if there's also distortion or ring modulation stages where some of the energy above hearing range might spill into audible sidebands if they're not nyquist-limited first.

[Blackthorn]

Yeah, that's the "rest of the synth" part that's more vulnerable to aliasing.

There's some ways to do band-limited distortion but...they aren't nearly as widespread, easy, or universal as band-limited oscillators.

Ring modulation is funny though because you'd ideally want the sidebands to modulate down by default rather than filter them out, that's why you're using it.