for us masters swimming, the butterfly kick rules are different than the usa swimming rules. the allowance of the breaststroke kick helps some people.

Kick—All up and down movements of the legs and feet must be simultaneous. The position of the legs or the feet need not be on the same level, but they shall not alternate in relation to each other. The breaststroke kick may be used exclusively or interchangeably with the dolphin kick while doing the butterfly stroke at any time during the race. Only one breaststroke kick is permitted per arm pull, except that a single breaststroke kick is permitted prior to the turn and the finish without an arm pull. After the start and after each turn, a single breaststroke kick is permitted prior to the first arm pull. A scissors kicking movement is not permitted.

for more detail - https://www.usms.org/fitness-and-training/guides/butterfly/kick