My workplace mostly uses C# for writing code. I'm trying to figure out a good way to indicate that a class with referentially transparent methods is intended to be referentially transparent (i.e., given the same set of arguments, you will always get the same return value) for future maintainers. I've considered just using static classes for classes that are intended to only contain referentially transparent methods but received pushback from my colleagues as they prefer an approach to unit testing where all classes depending on are stubbed out which they use constructor DI to accomplish. I don't see a disadvantage to not stubbing out referentially transparent functions for unit testing since they are almost always in-memory anyway but I've been unable to convince my colleagues of that.
Our class names use typical OO pattern names like DTO or Factory or Controllers and Models for namespaces. I'm not aware of any pattern names in OO that would indicate the sort of intent I am going for (i.e., to not introduce side-effecting operations or depending on things that read out of the db or filesystem).