Linked data from upgrade after VACUUM FULL not deleted.

Hello.

Few months ago we did successful pg_upgrade --link from 9.6 to 10. I did a
VACUUM FULL of all database this weekend and data from 9.6 directory were
not released.

I have tablespace with only one database.

PostgreSQL 10 folder has similar size to actual database size (by
https://wiki.postgresql.org/wiki/Disk_Usage it is 660GB):
681G ./postgres_raid_tablespace/PG_10_201707211

PostgreSQL 9.6 folder still has a lot of data in there:
547G ./postgres_raid_tablespace/PG_9.6_201608131

I did a really naive check if any links are still present:

ls -la . | grep "\->"

And nothing was found.

I have checked also which files are waiting to be deleted via lsof -u
postgres, but nothing was found in those directories.

I'm wondering if I understand this well and this is expected state where
old folder is not touched anymore and that's the reason why VACUUM FULL is
not releasing space in there.

Is there any way how to check if anything in that folder is really not used
anymore and consider that safe to delete?

I'll appreciate any suggestions.

Josef