> I thought the conditional was dependant on the method being present? So it could call it on 2.99, if not they can change that easily enough.

Nope:

https://github.com/tpope/fivemat/blob/e5fcb1085d89e2c8c477d7b08b821fb70861d490/lib/fivemat.rb#L15-L26

That approach is completely reasonable.  Sure, we could ask them to check `respond_to?` instead but I think it's asking a lot to make formatter authors change code specifically for 2.99...and there's no way we're going to be able to ensure that _all_ 3rd party formatters do this properly.