The PS Core breaking change guidance is here: https://github.com/PowerShell/PowerShell/blob/master/docs/dev-process/breaking-change-contract.md

Removal of '-Encoding Byte' is a breaking change from Windows PowerShell. Although certainly a cleaner implementation, it does not meet the bar for breaking changes as identified by the guidance above.

Even if fixing this causes a breaking change for PowerShell Core, that would be far less impactful than the breaking change for users coming from Windows PowerShell.