Yup. I have a 4k 27" (160 ppi), and I've kept with High Sierra because it's the last OS to natively implement subpixel text rendering. But I'll need to upgrade my OS soon, which means I'll need to switch to a 27" (or larger) Retina (220 ppi) external monitor.
As you said, what the OP is experiencing is not directly due to the hardware (the M1 Mini). Rather, it's likely because he was using High Sierra or earlier with his previous computer, and switching to the Mini forced him to use a newer OS, thus causing him to lose the subpixel rendering.
In addition to the Wikipedia article you posted, here's another good source.
It shows that subpixel text rendering nearly triples the horizontal pixel density of a display. So, with subpixel rendering, my 160 ppi monitor stays 160 ppi vertically, but effectively becomes ~480 ppi horizontally. Now that's sharp! Indeed, the difference it makes is so substantial that, from what I've seen, the newer OS's on a 220 ppi Retina screen don't look quite as crisp as High Sierra does on my 160 ppi monitor.
I'll be heading into the Apple store soon to pick up a repair, and when I do I'll check out the 254 ppi displays on the new MBPs to see if those seem significantly sharper to me than the 220 ppi on the external displays. What I'd really like is to be able to run the 32" Dell 8k (290 ppi) at 3 X scaling. Now that would be beautifully sharp, with a nice UI size. Alas, the Mac doesn't support the Dell (or any 8k for that matter).
A complete and clear explanation of Microsoft's 'ClearType' Technology -- including a freeware demo application.
www.grc.com
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