Docs should describe how the tool attempts to recover from legacy options, including any options which will be dropped on restore.
The fix in
TOOLS-2379for old indexes relies on applyOps, which isn't allowed by the MongoDB Atlas shared tier proxy. To address this, mongorestore should ensure that only valid, supported index options are sent. A white list of options may be found in index_descriptor.h (though not all appear to be documented options).
Open question: should this behavior require a flag? Or should it be a fallback only if the initial creation errors? The goal either way should be to allow someone to use a modern mongorestore to restore a legacy dump to a legacy server.