How many implementations do you care about? All major phones do very well at QR scanning these days including uppercase URLs.

Leaving a naked domain name like "www.example.com/1234" was not quite as good, but at least iPhones, Pixels and Samsungs worked well IIRC.

I used this trick to allow printing of QR codes at smaller resolutions for 4+ years, and phones have gotten noticeably better in that span at handling QR codes printed at smaller sizes, with uppercase, nonstandard shapes, borders, you name it.

If you make https: lowercase and the rest uppercase, with QR encoding that does smart segmentation, I'm pretty sure you can still get most of the benefit... but, exercise for the reader.

The smallest v1 QR code is 21 modules. That can fit 20 uppercase letters.

The 25 module size, is not that much bigger and can get 38 uppercase or 26 lowercase letters.