https://github.com/SeleniumHQ/selenium/blob/1bcb948894767cf4525b565f0c5dac73cd3e15e0/dotnet/src/webdriver/IHasDownloads.cs#L32C9-L32C9

List<string> GetDownloadableFiles(); allows add new items to the list.

I think it would be better to use IReadonlyList<string> or IEnumerable<string> (or any other immutable interface friendly with Linq). It is breaking change, but would not be widely spread.