StableHLO uses the MLIR Bytecode Format for serialization.
The MLIR Bytecode Format is a serialization format used to encode MLIR programs. From the MLIR RFC, it was built for "the benefits that a binary format brings to the table; namely serialization speed and size, mmap capabilities, more easily enabled versioning, etc." Performance, serialization size, and memory tests were run using large test from various dialects to validate the format.
MLIR bytecode was not specifically built to make MLIR stable, but the MLIR RFC notes that it would be possible to provide compatibility guarantees on top of this format, which we successfully did for StableHLO (see compatibility.md).