This was added in as part of 2.9.5 due to some user confusion over an API change. I think, it's hard to follow. The commit c6ddeb7 didn't reference a specific issue, and added a comment "eventually make this a deprecation warning and remove it." Four years later I think it's safe to revisit this.

Remove Context._legacy_resolve_mode. Possibly make _fast_resolve_mode the default, I'm not clear if it still needs to be a toggle with legacy mode removed. Investigate the issues that led to adding this to make sure we properly communicate the change in the changelog this time.