You can’t select an earlier version or target D365 version when you export the unmanaged solution in v9.0. The solution can import into same target version v9.0 or above.
In D365 v8.2 user have the option to select any specific target version, but this removed from v9.0.
You will get an error message if the solution is exported from v9.0 and trying to import the solution into v8.2 or less.
Dynamics 365 and Azure Solutions 