Questions on wal_retention_policy / removal of WAL files that are not required

[phebing]

The Documentation says

IMPORTANT: Currently, Barman implements only the full point in time recovery scenario, by constraining the wal_retention_policy option to main.

Does that mean main is currently the only "option" available, or does it mean "if you want to use the full point in time recovery scenario you have to set wal_retention_policy to main"?

In my scenario I need to be able to recover to the 1st day of the past 18 month and only in the last month a point in time recovery would be nice to have, but not required.
Since I expect the WAL Files to become huge (even when compressed) it would be nice to be able to delete them and set wal_retention_policy to 1 month.
If wal_retention_policy = main is the only option that would mean a retention_policy of 18 month would also keep all WAL files of 18 month, right?

And since setting up WAL archiving is necessary for streaming and rsync backups that means that there is no way to not have any (point in time) WAL if you dont need them?
(And yes I know that I need the WALs that are generated while a backup is being performed and pg_basebackup with option -X stream could give me a usable backup including those. But Barman does not give me such an option as far as I can see it in the docs)