Quite interesting, thank you.

However, in this specific instance, even if the text cannot be changed, couldn't the error itself in the server be processed and signaled differently, eg. by returning a Status Code 413[1], since clients ought to recognize that status code anyway?

[1]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/413

Since the caller gets this as an error object, instead of as a plain string, it seems likely that this is within the same process, i.e. a library function returns the MaxBytesError to a level higher in the business logic, without a network transmission inbetween.