I think they delete their accounts

Almost they do not follow terms and conditions of ResearchGate. Accordingly, RG deletes such profiles or misusers.

Most probably they are deleted by the owner

I think they do not want to be on ResearchGate and/or they did not follow ResearchGate conditions.

Prof. Nada Farhan Kadhim: Kindly, please let me add another reason to those mentioned by our respected colleagues. If you look at the attached snapshot, you will find another reason which says "If you want to change your email address".

You can reach this case by choosing "Account settings" which is the second option of the "Account settings".

Nada Farhan Kadhim most of the experts have already answered and yes there are different reasons the one being the researchers have either deleted their accounts or blocked by Research Gate and that is the most important reason why u see these statements

Most probably they are deleted by the owner