  With Ethereum's transition from Proof-of-Work to Proof-of-Stake in September
2022 came another paradigm shift, the Proposer-Builder Separation (PBS) scheme.
PBS was introduced to decouple the roles of selecting and ordering transactions
in a block (i.e., the builder), from those validating its contents and
proposing the block to the network as the new head of the blockchain (i.e., the
proposer). In this landscape, proposers are the validators in the
Proof-of-Stake consensus protocol, while now relying on specialized block
builders for creating blocks with the highest value for the proposer.
Additionally, relays act as mediators between builders and proposers. We study
PBS adoption and show that the current landscape exhibits significant
centralization amongst the builders and relays. Further, we explore whether PBS
effectively achieves its intended objectives of enabling hobbyist validators to
maximize block profitability and preventing censorship. Our findings reveal
that although PBS grants validators the opportunity to access optimized and
competitive blocks, it tends to stimulate censorship rather than reduce it.
Additionally, we demonstrate that relays do not consistently uphold their
commitments and may prove unreliable. Specifically, proposers do not always
receive the complete promised value, and the censorship or filtering
capabilities pledged by relays exhibit significant gaps.
