> The only major advantage of PNaCl vs early WASM was that PNaCl supported shared-memory threading right from the start (this is still knee-capped in WASM because of the COOP/COEP response header requirement).

Presumably that is because PNaCl predated spectre (?)

Indeed, NaCl and PNaCl would have been hit by Meltdown/Spectre at least as badly as SharedArrayBuffer.