I don't have a source, but no, I remember hearing about them before Slack's reactions were released. What they really were is a take on ("copy of") Path's reactions[1], which are very similar in concept.
But I think putting too much importance on "copy" is wrongheaded. Facebook's team seems smart to "copy" these things from other platforms that prove their worth, just like tons of other platforms "copy" original things that Facebook's team has vetted. Who cares.
I remember reading on multiple occasions (most recently, about two or three months ago) that "Mark Zuckerberg is preparing a dislike button", when, in fact, he was referring to adding more buttons to use in situations where "liking" things is not appropriate. This change brings just that.
Got a source for that? I've taken a quick look and can't find anything mentioning these before late 2015. Slack's blog post was in July (http://slackhq.com/post/123561085920/reactions)
But I think putting too much importance on "copy" is wrongheaded. Facebook's team seems smart to "copy" these things from other platforms that prove their worth, just like tons of other platforms "copy" original things that Facebook's team has vetted. Who cares.
[1]: http://techcrunch.com/2015/10/08/with-reactions-facebook-sup...