While blocks can already be added to the minecraft:climbable tag, the speed at which a player can climb the block cannot be modified. Being able to adjust the speed of a climbable block, especially based on the climber or the block's state, would be useful in extending the climbable tag provided by vanilla. While #62 already implemented this, the modifier would only be applicable if the block also has the minecraft:climbable tag.

This is actually something I was covering in an experimental fablabs project I call block-extensions. Hopefully in a week or two I'll have something to resolve that.