First I must point out that this question is somewhat unclear. What do you mean by "post a comment of more than 600 characters (...)"? (Emphasis mine.) If "comment" means that I would post it in the comments below the question, but there is no space, then obviously this has never been OK. It is even somewhat self-contradictory, considering the role that comments have.
By the development of the discussion, I take it you mean by "comment" just that it is not a complete answer, and I'll assume this to be the case for my answer.
This is somewhat tricky. The short answer is: it depends on how useful the answer is.
The long answer is that in general, the network allows (and wants) answers to be useful and relevant. It is the case that in Mathematics those two are heavily correlated with correctness and completeness. So the "safest" way to make a useful answer is just to make a complete and correct answer. Others which usually can also be OK can be summarized in "partial" and "incomplete", and I'll clarify the distinction I make. (The way I use the terms may be nonstandard, which is why I'm clarifying.)
By "partial answers" I mean answers that solve a problem considering some simplifying hypotheses. For example, a question which asks "Is every foo a bar?" can have a useful answer "If the foo is also a xyzzy, then this holds because of (...)". How useful that is obviously depends on the question, on the restriction and the context, and will be evaluated by the community. (E.g., if someone asks "Is every continuous real function $f$ on $[0,1]$ integrable", then responding it with "Yes if we assume it to be constant" is obviously not useful. )
By "incomplete answers" I mean answers which make significant progress towards a solution but do not quite manage to close it up. Again, how useful that is will depend on the question, the context and will be evaluated by the community.
Those kinds of answers can be OK. Again, it depends and will be evaluated by the community. As always, these evaluations can be contested by the community. Preferably, all this should happen in a reasonable way, no matter how outrageous one thinks something is.
If an "answer" is not any of those, then it will usually be the case that it is not even an answer. And answers should be answers. (I know this sounds facetious, but there is no simpler way to put it. There is a flag for something being "Not an answer" for a reason.) So it is most likely not OK to post "something" as an answer which is not a complete and correct answer, or a partial answer, or an incomplete answer. (There may be exceptions, but I cannot think of one.) For example, we frequently delete things posted as "answers" that are questions about other answers, or corrections about other answers etc.
Now, turning to the specific case of your answer. The first version of your answer is not a complete and correct answer, not a partial answer and not an incomplete answer. (As per what I defined above.) This already points to it being potentially not useful. But more specifically, it was just some computations. It is entirely reasonable to be considered not an answer, and as such, should have been flagged as such and elaborated upon or deleted. To be honest, I say this almost objectively.