GitHub never would have had that problem in the first place if :+1: were not so easy to do. There's no reason to make emoji part of GitHub flavored markdown or for it to be an autocomplete.
There is now no longer a reason to make emoji as part of the markdown, as the vast majority of devices can now do it natively. When Github flavored markdown was first created, emojis were most certainly not at all widespread natively.
+1
;)