GIMP Version: 2.10
Operating System: Linux
GIMP Experience: Experienced User

This is a view from 20,000 feet. I am not an expert.
But this came up in another thread, and I was surprised, on my first search, to not find a topic with this particular subject here in GimpChat. So I take it upon myself to start. Experts, feel free.

On my 2nd search for "GIMP Legacy vs Default difference", this time outside GIMP, I found this post where more literate user reported creative problems encountered in moving from 2.8 to 2.10, and the comments following from Elle Stone, a GIMP developer, as well. The problems were real, and this conversation between artist and developer no doubt led to improvement.

In essentials, Stone's comments translate to:
"People who necessarily had to resort to workarounds to get the sophisticated effects they desired from the 8-bit-based 2.8 version of GIMP are only hurting themselves by continuing to rely on tricking GIMP in 8-bit mode and refusing to change course to take advantage of the infinitely greater possibilities for effects possible with 32-bit software." (Well, it'll seem infinite...)

And so, to the quotes from Elle Stone with my emphasis:
For people who’ve always painted using 8-bit images and so necessarily perceptual RGB instead of linear RGB, getting used to working with linear RGB takes some time. The Blender/VFX/3D people started making that change many years ago and after sufficient ranting and user education and changes in software, finally it’s easy to use linear RGB in the relevants softwares.

It’s odd how long it’s taken artists/photographers and painting/editing software to start using/allowing painting/editing using linear RGB. But after years of everyone “doing it wrong” because of 8-bit software limitations, switching to using linear RGB really is an adjustment, which is the point of GIMP-2.10’s “legacy” blend modes. And sometimes artistic choice trumps “technically correct”, which is why usually in GIMP-2.10 there’s a way to “do the technically wrong thing for the right artistic reason”.

The whole point of high bit depth GIMP 2.10 is that unlike 8-bit GIMP-2.8, GIMP-2.10 operates on linearized RGB whenever appropriate to avoid “gamma” artifacts. Most GIMP operations do use linear RGB except of course for “legacy” mode. The few operations that don’t operate on linear RGB, use perceptual RGB for various good and carefully considered reasons. For example, Posterize is supposed to group tones “perceptually”, not linearly. Some operations make it really easy to operate on linear or on perceptual RGB at the user’s choice. And most operations do allow to choose one way or the other, but “how” isn’t always obvious.

The upshot is: Folks who have accustomed themselves to poking themselves in the eye to "make GIMP do the right thing" could very well benefit from learning how to cooperate with newGIMP to get what they want without so much eye-poking.

That is, if they have time to learn something new--and that ain't always going to be the case. However, those folks should consider that sometimes with some softwares the developers just kick out the crutches and force users into the new paradigm. Learn it now or learn it later. (Me? Oh, I'm definitely in the latter later kind of fellow.)

Cheers!

Still, I do think the Subtract mode operates differently between the two...