Encrypt old Time Machine backup for archival purposes?
Warning: I haven't tested this procedure, and there's at least one report of this converting the volume to APFS, which might cause trouble. So I can't really recommend this, I'm just posting it for the record.
An encrypted Time Machine backup is just a regular Time Machine backup stored on an encrypted volume (or an encrypted disk image, if it's on a network server -- not relevant here). Therefore, you can convert an unencrypted TM backup to encrypted just by encrypting the volume it's stored on. For some reason, this option isn't available in Disk Utility, but you can do it in the Finder by right-clicking (or Control-clicking) on the volume, and choosing 'Encrypt &quot;volume name&quot;'. You'll need to enter an encryption password (be sure not to lose this!), and then leave the disk attached for possibly quite a while as it converts the entire thing to encrypted format.
Note that @Allan's comments about TM not really being suitable for archiving still apply, whether the backup is encrypted or not.