"In JavaScript, neither weak references... is generally available". I think that was true with the old weak collation classes, but doesn't the newer JS WeakRef provide proper weak references?
WeakRefs landed around 2020 (in Chrome 84), but SqueakJS was started in 2013 -- "November 2013 Project started (after seeing Dan's Smalltalk-72 emulator at Hackers)" -- and that paper was written in 2014.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...
https://freudenbergs.de/vanessa/publications/Freudenberg-201...
https://squeak.js.org/