Add EIP: Elected block proposer has not been slashed

EIPS/eip-6987.md

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+---
+eip: 6987
+title: Slashed validator cannot be elected as a block proposer
+description: Prevents a slashed validator from being elected as a block proposer
+author: Mikhail Kalinin (@mkalinin)
+discussions-to: <URL>
+status: Draft
+type: Standards Track
+category: Core
+created: 2023-05-04
+---
+
+## Abstract
+
+Introduces a modificaiton to the consensus layer specification which ensures that slashed validator cannot be elected as block proposer.
+
+## Motivation
+
+A block proposed by a slashed validator is invalidated by the corresponding check in the [`phase0/process_block_header`](https://github.com/ethereum/consensus-specs/blob/3115d1140b23dd4c9c23fbd9e2428186cf816bde/specs/phase0/beacon-chain.md#block-header) function as it is defined in the consensus layer specification.
+
+At the same time the definition of the [`phase0/compute_proposer_index`](https://github.com/ethereum/consensus-specs/blob/3115d1140b23dd4c9c23fbd9e2428186cf816bde/specs/phase0/beacon-chain.md#compute_proposer_index) allows for a slashed validator to be elected as a proposer. This contradiction may effectively lead to an empty slot if a block is supposed to be proposed by a slahed validator.
+
+An impact of the proposed fix in the case of a single slashing on Ethereum Mainnet is negligible, while it becomes significant in the case of correlated slashings. For instance, a correlated slashing of `1/10th` of a validator set can lead to `1/10th` of missed proposals during a number of epochs after the slashing.
+
+## Specification
+
+The proposed change is specified in [the consensus layer specification](https://github.com/ethereum/consensus-specs/blob/82cd2f1f2317f664b7e179acdd898a327e1fe77e/specs/deneb/beacon-chain.md#modified-compute_proposer_index).
+
+_TODO: move specification to `/_features` and provide a link._
+
+## Rationale
+
+### Modifying `get_beacon_proposer_index`
+
+This function is modified to read a proposer index from a beacon state if a slot of a latest block header is the same as the `state.slot`.
+
+This modification is done to make the function return correct proposer index in the case when the proposer of a given block is being slashed during processing of the block.
+
+## Backwards Compatibility
+
+This fix changes proposer election mechanism in a backwards incompatible way and requires a hard fork to be deployed.
+
+## Test Cases
+
+_TODO: provide a link to the tests written as a part of consensus layer specification._
+
+## Reference Implementation
+
+_TODO: provide a link to consensus layer specification._
+
+## Security Considerations
+
+There are no observed security issues introduced by the proposed change.
+
+## Copyright
+
+Copyright and related rights waived via [CC0](../LICENSE.md).