I've seen it written, but it should only be used in informal contexts or when you're trying to describe how a person pronounces the words bye-bye.
To me, this purposeful misspellings usually falls into one of several roles:
- Parodying someone who speaks "cutely"
- Parodying an accent
- Parodying someone who is speaking insincerely (for example, using "buh-bye" to get rid of another person)
- When the writer wants to sound "cute"
These are the associations or connotations it has for me, anyway.
Nonetheless, people could use it sincerely, just for the sake of variety or to make it more personal, without intending it to be an affectation. In this case, it should still be reserved for informal contexts.