Thanks a lot for pointing out that Y is not always a vowel. I didn't know that because English is a foreign language for me. Although wikipedia says that in English language Y is a vowel and SELDOM is a consonant. Anyway, I was wrong bringig up that question, and the Author should have probably clarified that. Either way, test cases are pretty obvious about that, so it is not a big deal.

It's already been raised and answered, for example here: https://www.codewars.com/kata/57d1f36705c186d018000813/discuss#60211996424423002acea648

For C#: Tests are not correct. They don't consider "Y" as vowel. Therefore they force all the solutions to exclude "Y" from the replacement pattern.

I can't believe people upvoted this solution as the Best Practice and Clever... IT MUTATES THE ORIGINAL LIST!
Someone should add such check in Tests.