Spoiler alert on this thread is like adding the NSFW/NSFG tag to certain threads.
If the thread title is a specific show, of course people are discussing it in there. There is a virtually guaranteed chance you will hit a spoiler if you enter. It does not require a warning tag on the title, it requires common sense and some related decision making on whether to enter or not.
Adding spoiler alert to a show/movie specific thread title is like a net around a trampoline or "hot beverage" warnings on coffee cups. It absolves a lack of common sense from accountability.
I say let common sense prevail! Your lack of foresight is not someone else's fault.