Are there like any use-cases for it other than "well, I can add another drive to extend partitions whenever I want"? 'Cuz that's how it's often described (if at all), and it doesn't really make much sense to me. Like, if I install on a laptop, it'll most often have just one drive, so lvm seems unnecessary, and if I make some server-like setup with multiple drives, I'd go with some kind of raid with redundancy instead of just stitching the drives together (or mb yolo and raid 0).
If I remember correctly, arch wiki for example, had it used in the partitioning guide for dm-crypt without explaining the benefits against just luksformating /
OK, I haven't played with btrfs, but maybe the problem is that subvolid is used: https://wiki.archlinux.org/title/Btrfs#Mounting_subvolumes (the last sentence of the 1st paragraph), although I'm not sure, since there's also a regular subvol. Anyways, mb worth investigating.
So, what's in the fstab, anyways? Full contents uploaded on pastebin or smth similar would be helpful. Same goes for the bootloader used and the method of installation (the proper way or archinstall)
Anyway, you can boot into the arch iso, make mounts and re-genfstab -U it.
It's more like when you shut the laptop down, then turn it on only to be greeted with such message. So, I also haven't seen much of those back when, but only due to the unhealthy habit of maximizing uptime.
My guess is that's more like in SQL: you can shout if you want to, but DBMSes also understand lowercase