Since fa.traverse_(fun) and fa.sequence_ are semantically the same as fa.traverse(fun).void and fa.sequence.void I would propose to add aliases traverseVoid and sequenceVoid for them correspondingly.

Why?

In two words: discoverability and clarity.

The names traverse_ and sequence_ look cryptic and not self-explaining at all.
I personally witnessed guys who were relative new to Cats and they were able to figure out regular traverse and sequence theirselves but were struggling with the traverse_ and sequence_ variants. Most likely this is because across the industry underscores in method names are often used to mark some kind of internal/system API which should not be normally in use.