It was simply for convenience that people during the Grothendieck era wrote "=" to mean "isomorphic with a given or canonical isomorphism". Any other symbol would have had to be entered by hand in the typed manuscript, and we would have had to hope the publisher had the symbol. Once TeX became available, I switched from "=" to "\simeq" with the exact same usage. Of course we didn't actually mean equals! We assumed, correctly at the time, that readers would be intelligent enough to understand what we were saying. Little did we realize that fifty years later someone would be haranguing us because their computer didn't understand our writing. (See Buzzard, arXiv:2405.10387.)