Of the two proposals, Athens A sought to increase the gas limit and reduce the roll size required to bake from 10,000 tez to 8,000 tez. Athens B sought to just increase the gas limit. Athens A was autonomously activated onto the protocol in May 2019.
Notable changes included a new consensus algorithm variant (Emmy+), addition of new Michelson features to aid smart contract developers, an account rehaul that enabled clearer distinction between tz and kt accounts, as well as refinements to the quorum formula and addition of a 5% proposed quorum threshold.
Babylon was autonomously activated onto the protocol in October 2019.
Notable changes included increasing the gas limit per block and operation by 30%, improving the accuracy of the formula used for calculating baking and endorsing rewards, as well as several minor improvements to Michelson.
Carthage was autonomously activated onto the protocol in March 2020.
Notable changes included improving the performance of the Michelson interpreter, improving gas costs by adjusting the gas model, reducing storage costs by 4x, and various other small fixes.
Delphi was autonomously activated onto the protocol in November 2020.
The Edo proposal was injected in November 2020 with contributions by Nomadic Labs, Metastate, and Gabriel Alfour.
Edo adds two major features for Tezos smart contracts
Sapling and BLS12-381 to enable privacy-preserving smart contracts
Tickets for native on-chain permissions and asset issuance
Among other features, Edo also updates the Tezos amendment process by lowering period length to 5 cycles and by adding a 5th Adoption Period. A full changelog is available here