It is not "heavily" discouraged. But you have to pay extra attention to perform a clean copy of the borg repo's files, and ideally check both instances regularly for integrity. I would assume it's easy to forget validating the "cold storage" copy of your borg repo in practice.
https://borgbackup.readthedocs.io/en/stable/faq.html#can-i-c...
It is not "heavily" discouraged. But you have to pay extra attention to perform a clean copy of the borg repo's files, and ideally check both instances regularly for integrity. I would assume it's easy to forget validating the "cold storage" copy of your borg repo in practice.