[boricj]

Technically the ESP doesn't have to be a FAT32 partition. UEFI has file system drivers installable at runtime [1], so you could have an ESP partition that isn't FAT32... assuming that the file system driver for it is loaded. UEFI implementations must have built-in support for FAT per the specification, but you'd need to slipstream any extra EFI drivers into your platform's firmware that are required to get to the ESP.

I think that ESP partitions formatted as HFS+ were a thing on Intel Macs, but I've never had one to try it out.

[1] rEFInd's website lists a bunch of available file system drivers: https://www.rodsbooks.com/refind/drivers.html#selecting

[NekkoDroid]

IIRC FAT32 is the only that needs to be supported for a valid UEFI implementation. So to actually have something like that alpine image work for most it does need to be limited to that.

There are also standalone EFI drivers that can be loaded by other means: https://archlinux.org/packages/extra/any/efifs/ (e.g. systemd-boot loads these if they are installed to `<esp>/EFI/systemd/drivers/`)

[actionfromafar]

I guess one could do something like put a large ext4 file image on the FAT32 partition and mount that from within Linux.