IPv4 addresses are 32-bit and live in A records. IPv6 addresses are 128-bit and live in AAAA records. A dual-stack hostname has both. Happy Eyeballs (RFC 8305) tries IPv6 first with a short delay before IPv4. If your AAAA points at a firewall that drops 443, some users wait on a timeout instead of connecting immediately over IPv4. That presents as “the site is slow on mobile” rather than “DNS is wrong.”
Mail and IPv6
If an MX target has AAAA, senders may deliver over IPv6. Your SPF should include ip6: for those egress addresses. Reverse DNS for IPv6 uses ip6.arpa nibble labels — the PTR tool builds that name.
Publishing only A
Still fine. Do not invent an AAAA to look modern. Add AAAA when the host, the firewall, and the certificates all listen on IPv6. Check both with A and AAAA lookups.