> V8 shipped the import assertions feature in v9.1. This feature allowed module import statements to include additional information by using the assert keyword. This additional information is currently used to import JSON and CSS modules inside JavaScript modules.
> Since then, import assertions has evolved into import attributes. The point of the feature remains the same: to allow module import statements to include additional information.
> The most important difference is that import assertions had assert-only semantics, while import attributes has more relaxed semantics. Assert-only semantics means that the additional information has no effect on how a module is loaded, only on whether it is loaded.
NB: The above quotes have been edited to just the most relevant notes. See the link above for more information.
That page actually explains the change:
> V8 shipped the import assertions feature in v9.1. This feature allowed module import statements to include additional information by using the assert keyword. This additional information is currently used to import JSON and CSS modules inside JavaScript modules.
> Since then, import assertions has evolved into import attributes. The point of the feature remains the same: to allow module import statements to include additional information.
> The most important difference is that import assertions had assert-only semantics, while import attributes has more relaxed semantics. Assert-only semantics means that the additional information has no effect on how a module is loaded, only on whether it is loaded.
NB: The above quotes have been edited to just the most relevant notes. See the link above for more information.