That seems weird given NIST, and the US Government, set a requirement for IPv6 Only back several years ago, and it sort-of became part of the JWCC requirements (It wasn't in the requirements, IIRC, because it came after those were set, but the government wouldn't fully approve use for JWCC if you didn't meet it).
You'd think they'd have sprinted for that feature as fast as they could go.
USG also set a whole bunch of security requirements under FedRAMP that Microsoft can never meet, but they received an ATO anyway because they are so heavily entrenched in government.
Especially given that it is now owned by Microsoft, which has been working on IPv6-only (at least on their corporate network) for almost a decade:
* https://blog.apnic.net/2017/01/19/ipv6-only-at-microsoft/
* https://www.arin.net/blog/2019/04/03/microsoft-works-toward-...