Tax calculator using functional programming
Hi and welcome to CodeReview.

You are right, your code does not modify state (has not side effects) and the output of the methods depends only from the input... Therefore I would guess that the functions are "pure functions".

However, I would prefere an IEnumerable of int instead of List of int as parameter type because with an IEnumerable the API shows that the method does not change the passed list.

If you value immutability, there are also some immutable collection in .net (e.g.: The ImmutableList).

If you like functional programming, F# is really worth a look! FSharpForFunAndProfit is a good point to start :D