Drop tables but space not claimed in postgres 12

I have upgraded Postgresql 9.5 to Postgresql 12.4 a few days back using pg_upgrade utility with link (-k) option. So basically I am having two data directories i.e. One is old data directory (v9.5) and the current one in running state (v12.4). Yesterday I have dropped two tables of size 700GB and 300GB. After connecting to postgres using psql utility I can see database size whose tables was dropped got decreased (with \l+ ) but what is making me worry is that only a few ~50GBs have been freed from storage partition. I have run vacuumdb only on that database but no luck. I have checked if any deleted open file is there on OS level using lsof but there is none. Note : Still I have not deleted old data directory (v9.5). I am not sure if deleting will impact my running postgres 12 as I have used 'link' option in pg_upgrade. Looking for the solution.