Alphanumeric mode can encode URLs, but it only supports uppercase characters, so no one uses it.
I found that they are still pretty useful because they're noticeably smaller grids. They can be enlarged to use up the same space on screen/print as a denser UTF-8 QR, thus be clearer and easier to scan. Some readers will assume that an all-uppercase URL is supposed to be lowercase and convert it, others will use them as-is. Use short URLs, like https://qr.test/thing so only the case of the path part matters, then make sure /thing and /THING go to the same place, and you're good.
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I found that they are still pretty useful because they're noticeably smaller grids. They can be enlarged to use up the same space on screen/print as a denser UTF-8 QR, thus be clearer and easier to scan. Some readers will assume that an all-uppercase URL is supposed to be lowercase and convert it, others will use them as-is. Use short URLs, like https://qr.test/thing so only the case of the path part matters, then make sure /thing and /THING go to the same place, and you're good.
Thanks for the informative guide.