Memory-64 and multiple memories are extension proposals for Wasm itself, which have not yet been standardised, although they are already at phase 3 of the Wasm proposal process. AFAIK, wasmtime (the engine used by the replica) already implements them, but it would be risky to start depending on them before their standardisation has been finalised (which would be phase 4). And that process is outside Dfinity’s direct control, it is blocked on other engine vendors implementing the proposals.
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