Commutative means that changing the order of the operands yields the same result. Ex: a+b = b+a. In that way, "alternative" is (mostly) commutative, but there are exceptions: Amazon is an alternative to a local bookstore, but a local bookstore is not necessary an alternative to Amazon.

The transitive property implies that if a=b and b=c, then a=c. "Alternatives" are also (mostly) transitive. A counterexample would be that Slack is an alternative to IRC, and IRC is an alternative to Slack, but Slack is not an alternative to Slack!

Let’s not forget the Windows Live Messenger. I guess it was more like Discord alternative of that time.