WPT details: There are existing WPT tests for constructable stylesheets (https://wpt.fyi/results/css/cssom/CSSStyleSheet-constructable.html?label=master&label=experimental&aligned&q=css%2Fcssom) which will be modified to match this removal.
The original spec for constructable stylesheets allowed for calls to sheet.replace("@import('some.css');"). This use case is being removed - calls to replace() will throw an exception if @import rules are found in the replaced content. The CSS Working Group resolved to remove this feature on Feb 19, 2020: https://github.com/WICG/construct-stylesheets/issues/119#issuecomment-588352418. This feature was deprecated in M83, and will be removed in M84.
There is an asymmetry between replaceSync(), which disallows @import rules, and replace(), which currently allows them. Additionally, work is underway on CSS Modules, which appear to be coalescing on also disallowing @imports for a first version, and (possibly) on a next version where @imports behave as leaves in the module tree. This would also be an asymmetry with the current replace() API. For these reasons, the CSS Working Group resolved to disallow @import in calls to replace().
Docs: https://github.com/WICG/construct-stylesheets/blob/gh-pages/explainer.md