What risks are there in allowing external clients to resolve internal IPs to their domain names? The server is used internally for clients, as well as for external clients needing to resolve a web server's domain. Couldn't allowing these reverse lookups allow an attacker to gather a wide array of information if the domain names contain usable information?
I've never seen a DNS server that does not allow reverse DNS lookups (PTR record lookups). Moreover, if a DNS server didn't allow reverse lookups, I think that could cause some things to break. For example, if one of the hosts were used as an outgoing SMTP server, and it was not possible to do a reverse DNS lookup on the IP address of that host, then this would cause mail delivery failures. See https://sendgrid.com/docs/ui/account-and-settings/how-to-set-up-reverse-dns/
Note that the PTR record (used for the reverse dns lookup) must be stored in the zone file for the IP address, not the zone file for the domain.