Our node implementation has full support for bip-0151.
Might be a waste of your time there, I expect it to change when it gets review and public implementation work.
Why are we seeing and speaking to Bitcoin Core nodes with bip-0151 support on mainnet but no source code in GitHub. The paper says "Reference implementation" and it is blank. Is this closed source development we are detecting on mainnet or is there a public branch with the code?
You're probably connecting to fake bitcoin core nodes, there are a lot of them mostly run by blockchain analytics companies, thanks for identifying a new distinguisher.