This finding is preventing us from using the GeneratedRegex attribute to generate optimal regex code.
I've been discussing this one with Microsoft and here is their response:
"The base Regex still inherits ISerializable because removing it would be a binary breaking change, but if you try to serialize/deserialize a Regex, you'll get a PlatformNotSupportedException. And Regex itself is not marked [Serializable].
We will not be making Regex or source-generated derivations [Serializable]."