From what I can tell from scripture, The "apostasy" has been going on since Genesis 3. Especially so by Genesis 4. And a casual read of scripture and historical records of the last few thousand years, I cannot see a time when there wasn't a time when many were separating themselves from God.
An example, Paul stated in Acts 19 that all Asia had heard the Word of the Lord. And later, Paul wrote to Timothy that all in Asia had turned against him. Seems like a major "apostasy" to me. The church couldn't even get out of the first century before there were problems. The letters to the 7 churches in Revelation sure doesn't paint a positive picture. For decades, most "christians" are just in name only out of family tradition. Most "christians", when national surveys have been done, don't believe Yeshua is the God they believe in. Most also say that Satan is a concept not a personage.
I am not convinced that "apostasy" (which is an anglicized word from the greek apostasia) has the meaning of "falling away" in the original. Many Greek scholars have made a compelling argument that apostasia simply means "departure". The only way to make it mean "departure from the faith" or "falling away" is to show in the context was is being departed from. Without that, it stands on it's own simply as "departure". Many early English translations such as the Geneva Bible used "departure", "a departure", or "the departure" to translate 2 Thessalonians 2:3. The Latin Vulgate used dicessio which suggests a physical departure, not a spiritual one. The only instance where apostasia is used elsewhere in scripture is in Acts. And there, what is being departed from is mentioned... (Moses - Law).
Given the history, I am not sure that a "apostasy" or "falling away" is really anything to hang one's hat on as a significant sign. But a physical departure could be. In 2 Thessalonians 2:1, the context of the passages is our gathering to the Lord. And evidently in verse 2, these folks had gotten a fake letter, as if from Paul, that the Day of the Lord had started. For them to be worried about this, they must have been taught by Paul that they would be delivered from that time on the earth and not experience it. Paul then reminds them that day would not come until there is a departure and then the man of sin would be revealed. He reminded them that he taught them this earlier in verse 5.
If they had been taught that they would go into and thru the calamity that comes upon the earth, and would see the man of sin (Antichrist), then why would they be upset that they were already in that time via a fake letter? Why would Paul need to set them straight? For them to be upset, it would imply that they thought it would be different and they wouldn't go into that period. And Paul reminds them of that.