Change scaling algorithm for non-retina apps running in low resolution on a retina display? Regarding non-retina apps running in low-resolution mode on a retina display:

In the past, circa 2012 (ref), these apps were scaled with nearest-neightbour algorithm, where 1 pixel would become a group of 4 identical pixels. Confirmed in Apple Docs from the period

At some point since then, at least by 2016 (ref), these apps are now scaled with a bilinear (or similar) filter meaning that a single pixel does not get scaled up cleanly and the resulting group of 4 pixels is blurred with surrounding pixels


So, my question: is there a way to change the scaling algorithm back to nearest-neighbour?
I don't have a system on which I can reproduce your issue, so this is just a guess, but check out the advice on this forum post: https://forums.macrumors.com/threads/change-osx-scaling-mode-for-retina-screen-with-non-retina-content.1428540/
Namely:

Try using SwitchResX to enable 1:1 mode at your display's native resolution (I believe this will make your content very small; not sure whether that's a workable solution for you)
Use an external, non-retina display :)