Been suggested way back 2012. Guess what? Even the Global Mod @hilariousandco already made a suggestion regarding the feature. I feel the way as well to have this spoiler tag but I don't think if this will be implemented here maybe on the developing epochtalk forum.
I just had a look at smf forum and searched for spoiler tag and found this thread-
Inline Hover Spoiler BBCode. This is created back in 2008 and updated in 2014 but sadly I can't see 1.1.19 (bitcointalk smf version) in compatible list. Maybe that's the reason?