Note: This writeup is for advanced users and developers. For a guide that will actually walk you through this, ios.cfw.guide is updated. guides.stkc.win should be soon™️.
So, I got bored and decided to do more research into alternates. Turns out you can still abuse it to update to 14.8 on iOS versions that don't support alternates. You're welcome.
This requires a jailbreak. Of course, you can do the backup editing stuff if you're not JBed but:
a. It's a wack method imo
b. I didn't try it so I'm not gonna explain it