Twitter changes behavior after submitting a research paper about it to conference. What to do?

Technology and technology based applications keep changing all the time, and frequently so.

Research methodology requires that you document these changes as they happen and annotate all references with dates. For example, you may add information that can act as a sort of disclaimer: For example:

N.B: As of March 30, 2017, Twitter discontinued its 140 characters limit for replies. At the time of submission of this paper, the 140 Characters limit was still in force.

Similar issues arise frequently in fields that change quickly due to political, legal or technological developments. In general, these changes don't affect the veracity of the study; they only affect its scope. So usually reviewers will ask you to address the change in the concluding section, perhaps through an informed speculation on how the change matters in the future. In other cases, where the change does not matter greatly (perhaps this is the case here), they will simply ask you to acknowledge it in a footnote or the like -- if they recognize it at all.

Does the character count affect the importance or relevance of your findings? You cannot help what the reviewer thinks, but a good reviewer should consider that although the technology has changed, the meaningfulness of your findings should be the same and potentially worthy of presentation at the conference. Also, for many conferences papers are distributed to volunteer reviewers and it is possible that whoever your paper is assigned to may not be all that familiar with Twitter anyway. Good luck!