ETJ de-annexations are done at the request of the land owner. So the de-annexations occur on a parcel by parcel basis. It is NOT that the entire ETJ gets to vote for removal and, if the majority vote is for it, then the entire ETJ is dissolved.
Edit/Clarification: IT CAN be done the way you described. If you live in a subdivision and think that the entire subdivision should be removed from the ETJ, you can submit through the appropriate process and, if you have all of the requirements met, the city will have to have it on the next election and all of your neighbors will have to approve it by majority to have the entire subdivision removed.
However, the VAST majority of what is happening now is that a single landowner petitions to remove his property and the City is granting.
There is a process the city can make the landowner go through to get approved, but it is so costly, time-consuming and so stacked in favor of the land owner that the Cities are simply just granting them their request for de-annexation from the ETJ.