I want to +1 to forbid new equality-delete writes in v4, keep them readable for backward compatibility. Equality delete imposes an asymetrics burden to the reader while simplifying the writer's work, also slows down and complicates the feature development.
For existing v2/v3 tables with equality deletes, upgrading to v4 should continue to be a lightweight, metadata-only operation, with no synchronous EQ-delete rewrite required. We have a working precedent for exactly this shape of change: how DVs were introduced and adopted in v3. A v2 table with existing position-delete files could upgrade to v3 without rewriting anything; those files stayed valid for reads. The spec requirement drove the engine change instead: writers had to produce deletion vectors for new DML going forward. As Huaxin proposed, converting the carried-over equality deletes into DVs can be a separate maintenance action. Thanks, Hongyue
