Ethereum now utilizes a proof of stake consensus mechanism, and underwent this migration from proof of work in a three phase process, beginning with the launch of the Beacon Chain on December 1, 2020.
This newly formed consensus chain ran mostly independently of Ethereum’s proof of work chain (execution layer), until The Merge on September 15, 2022 .
Technically, post-Merge Ethereum is still two tightly coupled blockchains. However the block trees of the execution layer and consensus layer have a 1:1 mapping, meaning that every EL block has one, and only one CL block which corresponds to it, and vice versa. This 1:1 mapping of EL and CL blocks means that we can think of post-Merge Ethereum as a single blockchain consisting of two layers.
Finally, withdrawals were enabled on the consensus layer on April 12, 2023, completing the final phase and creating an end-to-end proof of stake consensus mechanism.
Where the first three upgrades were sequential and crucial to enabling PoS on Ethereum, the remaining are supplemental upgrades and non-sequential, but in the case of SSF and Enabling More Validators, are highly dependent upon one another and intrinsically linked.
More validators means processing a greater volume of signatures and data in each slot, which can slow down the finality process.
Ethereum faces the technical challenge of efficiently coordinating numerous validators to achieve SSF (a potential boon for security) without sacrificing the network's decentralization (a high number of validators). Put another way, SSF would be much easier with small validator count,
but this would lessen one of Ethereum's primary strengths.
Research and debates are ongoing to determine the optimal path forward to strike the appropriate balance.