I have asked for like the ability to apply a filter that maps real strings from real users to a very small number of character classes, so that I may run some codec on data that is equivalent to user data for the purposes of the codec. I have heard, this is not a good use of my time, and if I really care so much I should instead make various guesses about the production data distribution of user-created strings (that need to be json-escaped) and then we can deploy each of them and keep the best, if we can even tell the difference.

If the real data is sensitive, it's hard to distill test data from it that succeeds in fully removing the sensitivity but that is still useful. Depending on the domain even median string length could be sensitive.