Inspired by the article at the end of my post here.

Officially, per the Russian Ministry of Defense website, it has a capability for 20 of the missiles. But there have been persistent rumors that only one or two tubes were upgraded to allow it to act as a test bed. The difference to the Russian Ministry of Defense information may reflect their status under New START (Strategic Arms Reduction Treaty). Under this all of the tubes have to be counted even if not used.

Since Mr. Sutton has shown in the past that he is familiar with the work of our esteemed expert /u/Vepr157, I suspect that the "persistent rumors" may come from Vepr157 or the same source that Vepr157 uses.

Vepr157 has said.

Yes. It's always seemed a bit odd to me. Why would you keep an expensive 48,000 ton submarine around just to use two missile tubes for testing?

She's just an experimental boat at this point. I believe only the forward two tubes can actually carry the Bulavas.

So my question is, how sure are we that Donskoi is nothing but a test bed anymore? And if it is, why in the world would the Russians, as Vepr157 says, keep one of the most expensive submarines in the world just for that purpose?

Perhaps this has been discussed to death, but I can't find where.

https://www.forbes.com/sites/hisutton/2020/08/23/last-typhoon-can-the-worlds-largest-submarine-still-destroy-the-world/