Technically it is available, but it's just not widely supported. Though it's really unnecessary. Brotli is, in many ways, very comparable or a near equivalent to zstd.
Common reason being someone forgot to check how many bytes read() actually returned and just assume it filled the buffer.
Robust data formats have double protection against this class of bugs with unambiguous SOF and EOF boundaries that the consumer can assert. I guess a sanity-byte once every X Megabytes wouldn’t hurt either.
I still don't understand the use case limitation, or exactly where this is a potential issue. Why is the data being interrupted? Is this some kind of raw UDP stream use case?