You do have access, just not through tor.
That is reduced access. And it makes a world of difference because the lost access also forces excessive disclosures. It would be perversely narrow to disregard that as a security compromise.
Also, you assume everyone has clearnet access, not just that everyone has the will to use clearnet, and that everyone would find clearnet appropriate for this, and that some users rightly see clearnet as a break from the rule of least privilege principle. But some people offer open internet access to the public on a tor-only network. Users on such a network have no clearnet option.
Furthermore, I personally have a DNS problem with my local public library. I have not yet taken the time to troubleshoot it, but when I connect to the library’s network, all clearnet attemps fail because of some DNS problem. Tor is the only way I can access the internet from my local public library. So until I get to the bottom of that problem, the IRS website is unavailable.
For me, not having privacy-respecting access is the same as not having access. For pushovers who don’t think about their own security, their availability is not affected. More broadly, it’s not your place to tell users what threat model and security posture is right for them -- unless they hired you for that. If a blockade forces a connection outside the parameters of someone’s security policy, they have lost availability.
Nor through Bluetooth. Nor plaintext. “Availability” does not mean you will support every known protocol so that purists and idealists can be happy.
You can’t dress this up as “neglecting to offer Tor support”. The IRS is taking a deliberate action that reduces availability. They took something that works by default and crippled/broke it in an act of sabotage.