To my knowledge, name constraints are very rare in actual use, and a bunch of implementations don't even support them. Am I wrong?

Rare in public PKIs, yes; not so much in private ones, I would imagine.

From my limited experience (using them at home) they seem to be fairly well supported in client libraries, actually!

- OpenSSL has full support, https://docs.openssl.org/master/man5/x509v3_config/

- GnuTLS has full support, https://www.gnutls.org/manual/html_node/X509-certificate-API...

- go (crypto/x509) supports everything except DirectoryName constraints, https://pkg.go.dev/crypto/x509#Certificate.Verify

- Python (pyca/cryptography) has full support, https://cryptography.io/en/stable/x509/reference/#cryptograp...

- Windows/Schannel has full support, https://learn.microsoft.com/en-us/windows/win32/seccertenrol...

- Java has full support, https://docs.oracle.com/en/java/javase/21/docs/api/java.base...

- I know that NSS also supports them from experience but (sorry!) couldn't find the docs in a few seconds of searching

- I know that Strongswan supports them well because I actually opened an issue about it a few years back and got it fixed :-)

(Just a few off the top of my head)