Hi @wendywick! Welcome to the forum.
This is an interesting case. The public suffix list is broken up into two sections: the ICANN section, and the PRIVATE section. The ICANN section mostly consists of exact TLDs, like com and org, plus effective TLDs like co.uk. We have certain obligations under the Baseline Requirements with regards to public suffixes, mainly that we can’t issue for *.foo where foo is a public suffix.
While those requirements don’t technically prevent us from issuing for TLDs, like com or co.uk, our policy so far has not been to issue for ICANN TLDs because it’s generally not a best practice to put websites on such TLDs. Partly that’s because, for the dotless TLDs, it can create confusion between local network hostnames and global DNS hostnames.
k12.wa.us is in a funny position because it’s listed as a TLD in the ICANN section, but it seems like it is actually being used as a website in addition to a TLD.
So, long story short, this is our current policy. We’re not likely to change it in the short term, but I do appreciate you bringing the instance to our attention since it will help inform our future thoughts on it. I’d recommend getting an inexpensive certificate from another CA if you’d like to continue serving k12.wa.us as a website.
Thanks,
Jacob