I want to know the answer to the following question:
Since what version can already existing characters no longer be added to the standard?
All the background I'm provided with is that "Unicode was designed to provide code-point-by-code-point round-trip format conversion to and from any preexisting character encodings, so that text files in older character sets can be naïvely converted to Unicode, and then back and get back the same file.". Is that enough to answer the question?
Answer: no