Retention policy of archive/ I have archive_mode = on and archive_command = 'cp %p /var/lib/postgresql/11/main/archive/%f'

What is retention policy of files under archive/? Will pg clean up it periodically or let it pile up until administrator panic and get involved?
The retention policy is whatever you decide on and then implement.

Putting this under your control is what archive_command is for.  If you don't want to be in control and want PostgreSQL to do it for you, then use replication slots or wal_keep_segments instead.