It's the reason Anthropic called their next release 3.7 Sonnet - the 3.6 version number was already being used by some in the community to refer to their 3.5v2.

Anthropic moved from 3.5, to 3.5(new), to 3.7. They skipped 3.6 because of usage in the community, and because 3.5(newer) probably passed some threshold of awfulness.

People also use 3.5.1 to refer to 3.5(new)/3.6.

The remaining difficulty now is when people refer to 3.5, without specifying (new) or (old). I find most unspecified references to 3.5 these days are actually to 3.6 / 3.5.1 / 3.5(new), which is confusing.

That's not correct. I have always referred to it as v3.6, and I've seen plenty of other people do so too. It's why their next model was called v3.7