Removal of the ExtendSectorExpiration method from the miner actor

[rvagg]

I'm proposing the removal of the ExtendSectorExpiration method from the miner actor.

This method has been effectively (not technically) deprecated since the introduction of ExtendSectorExpiration2 in FIP-0045, which provides the same functionality with greater flexibility and support for verified claims.

As of right now, on-chain analysis shows that the last ExtendSectorExpiration message was processed on December 15, 2024:

• bafy2bzacecmyltma2j42wai3vqymu6apmcervz3eia3oixhyyls2tdcvj7okm

This proposal follows a similar pattern to FIP-0101 (see discussion #1116), which proposes removing the ProveCommitAggregate method that was superseded by ProveCommitSectors3.

The proposed removal would help reduce technical debt and simplify the protocol without impacting current operations, as all functionality is already covered by ExtendSectorExpiration2. Storage providers and tooling have already migrated to using the newer method, making this a safe change.

It's important to note that this FIP does not remove the ability to extend sectors - it only removes the redundant legacy method while preserving full functionality through the more flexible ExtendSectorExpiration2 method.

Proposed FIP content: #1137