We're still transitioning to IPv6, so yes - for now - one needs an IPv4 address to reach the whole Internet.

That doesn't make "You can't run an email server on ipv6" correct.

Yes it does- imagine an email server that can’t send/receive email from everyone with an email address.

At best it’s broken at worst it’s not an email server

By that logic, basically nothing supports IPv6.

There's nothing that prevents email from working over IPv6. The sender and receiver just need to have IPv6 connectivity.

Every email provider that supports IPv6 will also support IPv4 (for the time being). To use Gmail as an example:

    aspmx.l.google.com. 293 IN AAAA 2a00:1450:400c:c0b::1a
    aspmx.l.google.com. 293 IN A 173.194.76.27

There is both an A and AAAA record. You can send emails over IPv6 or IPv4 to Gmail addresses.

Did you mean you can't exclusively use IPv6? Because that is also true for browsing the web - because so few websites support IPv6, a lot of links will just not work.

Yes, you must have an IPv4 address for your mail server for the foreseeable future unless you want to only be able to send to IPv6 receivers.