( I'm not sure if it is a bug or not but the warning text should be changed for that case. )
When loading a world in an unknown version, the game will warn you that the world was loaded in version unknown and ask you to backup or load.
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@Greymagic27 An unknown version is not an actual version it is just the game that can't figure out the world version in level.dat.
See also MC-158328
Very old versions (I think 1.7 and before) are unknown versions. Thus this is very likely intentional.
I would say that this is intentional. If you bought a Minecraft account before Mojang registered the dates of account creation it will also say "unknown".