I'd agree that the description of "M" in https://wiki.mozilla.org/Security/Server_Side_TLS#Recommende... is unrealistic for a site with a large and diverse audience, so far. The primary issue is that it requires turning off TLSv1.2.
Wikipedia is a good example of "about as Modern as you can get in the real world with a big global audience". It's a little stricter than "Intermediate", but doesn't quite meet the "Modern" description. The key items there are that Wikipedia still supports TLSv1.2 (but only with TLSv1.3-like ciphers) and it still supports dual cert compatibility (ECDSA+RSA). The RSA support is likely to be on chopping block Soon, as the only real use-case for RSA in this config is to support ancient installs of the last-known-good version of Chrome (49) on WinXP SP3, but Wikipedia will likely have to continue supporting TLSv1.2 for quite some time.
In any case, though, Wikipedia still gets an "M" rating in the check, so either the description is wrong or the check is buggy:
https://observatory.mozilla.org/analyze/en.wikipedia.org#tls