My understanding is that OP_RETURN was first introduced in v0.9.0 (https://bitcoin.org/en/release/v0.9.0).
This link (https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2014-November/006917.html) mentions "initially supposed to be 80 bytes, but got reduced to 40".
This link (https://github.com/bitcoin/bitcoin/pull/5286) says: "The maximum size for OP_RETURN outputs used to be 80 bytes, then got changed to 40 bytes to be on the safe side".
Pull request https://github.com/bitcoin/bitcoin/pull/3737 talks about reducing it to 40 bytes. Was it ever in the code at 80 bytes prior?