Table space not returned to the OS ?

Hi,

A few months back (October) I had upgraded a Postgres instance from v12 —> 14.

The database disk size under /var/lib/postgresql/12 was around 800GB+ back then.
Note, that IIRC I had used hard-linking during the upgrade.

In the database itself, lots of things have changed since.
In fact, that database itself has been dropped at some point and restored from a backup.

As I was running out of disk space, I started investigating and found out that

/var/lib/postgresql/12/main/base/16385 —> 886GB+
/var/lib/postgresql/14 —> 400GB

The last modification date on that directory (../12/) appears to be around a month ago,
When the table with relied 16385 was in fact dropped.

Now, In my update scripts (I use this db as an OLAP) I occasionally run VACUUM.

Is it weird that the 886GB space hasn’t been returned to the OS yet?

What’s the safest way to return it to the OS manually?

Thanks