I'm more concerned about obsolescence of LUKS
What about USB mass storage support? The underlying PATA or SATA disk access? (It's already pretty hard to find motherboards with PATA ports.) How about Linux itself, or USB for that matter?
As frostschutz said, it depends very much on why you are encrypting the main drive.
If your main concern is LUKS obsolescence, there's a pretty simple solution. Make a small unencrypted partition with some basic file system support for which is unlikely to go away (ext2, ext3 or maybe even FAT32) and put ISOs (live and install) for your favorite Linux distribution there, and devote the majority of the disk to an encrypted partition holding the actual backup. Store hashes of the ISOs to be able to detect corruption, and check them on a regular schedule to make sure the drive does not suffer from bit rot. If you need to, just boot from the ISO and mount the encrypted partition. This is trivially accomplished in a virtual machine. Be sure to try it out before you need it; backups are easy, restores are hard.
And major Linux distribution ISOs don't go away easily. Here is year 2000 vintage Red Hat Linux 6.2, which came with a 2.2.14 kernel. I'm sure you can find older if you look around a little.