Your question was not put on hold because it was asking about living persons, or events that occurred less than 100 years ago. These are not criteria that are used on this site to determine whether questions are on-topic. Your question was closed for the following reasons:
Most importantly, your question is not historical in nature, at least now in its current form. You asked what people (physicists) mean(t?) when speaking about the existence of certain objects. That means that you're asking about the philosophical implications of something "existing", in the sense that physicists use the word. That is an essentially philosophical question, namely about the philosophical views of physicists. The question might still be on-topic on physics.SE as well, since it is asking about the philosophical views of physicists, but I doubt it.
Your question seems to be asking for opinions on what some people "mean" when they say something: Questions that seek answers based on opinions are discouraged on this site. Furthermore, that is a kind of ill-defined and broad type of question.
Both of these reasons for closure of the question could be fixed by a thorough edit. If you are really interested in the historical aspects of the question, please edit your question to reflect this more clearly. But be very careful to clearly distinguish your question from a philosophical one.
Right now, it seems to me like your main interest is actually in the philosophical aspect of the question. If this is indeed the case, feel free to flag your own question and leave a custom message for the moderators, asking for the post to be migrated.