Although the CURRENT WordPerfect file format is about 18 years old (it was created, as a result of MS breaking the old 5 format) in 1994.
To the best of my knowledge, WordPerfect files are both backwards AND forwards compatible between v6.0 (released in 1994 as I said) and the latest version.
So incompatibility like this is a deliberate or accidental vendor choice, not something that is inevitable ...