As a counterpoint, Neptune and Pluto were "dark matter" prior to confirmation with observations. They were theorized to exist due to their perturbations to observed motion of known planets.
No, it wasn't, because, as you note, that observation was accounted for by adopting new laws of gravity, not by adding more matter to the distribution of matter we used in our model. Those are two different things.
By analogy suppose we find out in 35 years that some non-lambda-cdm, non-particle theory is correct? It would be revisionist to say we didn't believe in dark matter today.
If you mean that nobody considered modifying the theory of gravity to explain the precession until Einstein and General Relativity, that is not correct. By the 1890s pretty much everyone had given up on finding another planet and a number of astronomers were considering modifications to the law of gravity.
In any case, you are shifting your ground from the comments you have made elsewhere in this thread. Elsewhere in this thread, you have been claiming that "dark matter hypothesis" includes the hypothesis that we should change the laws of gravity instead of changing the matter distribution in our models. Now you are admitting that those are two different things, which is the point I have been trying to make all along.
> suppose we find out in 35 years that some non-lambda-cdm, non-particle theory is correct? It would be revisionist to say we didn't believe in dark matter today.
"Believe in" is the wrong term. Dark matter is a hypothesis (just as suggesting there was another planet causing Mercury's excess orbital precession was a hypothesis), which appears to be the one most cosmologists currently think will end up being the right one. But that does not mean anyone claims that it is the only hypothesis, or that that hypothesis includes the hypothesis that we should modify the laws of gravity. The latter is understood by all cosmologists to be a separate hypothesis. Just as modifying the laws of gravity to explain Mercury's excess orbital precession was understood by all at the time to be a separate hypothesis.
> you have been claiming that "dark matter hypothesis"
No. I have claimed that the class of dark matter theories includes those which reject the existence of "dark matter" under the (possibly incorrect) assumption that the historical term "dark matter theory" refers to any theory that explains galactic rotation curves, as coined by Zwicky and cataloged by Rubin. In general, "A theory of X" can certainly reject X, and replace it with something else.
And this claim is incorrect. "Dark matter" is a particular hypothesis for explaining the discrepancy in galaxy rotation curves (and other observations as well, such as features of the CMB that cannot be explained with a model that only includes the matter visible to us). It is not just another name for the discrepancy itself, or for any theory that claims to explain the discrepancy by any mechanism whatever.
> under the (possibly incorrect) assumption that the historical term "dark matter theory" refers to any theory that explains galactic rotation curves
And this is an incorrect assumption. That's what I've been saying all along. See above.