This release upgrades PostCSS to version 8 which results in better performance and enable further optimization in future. As it's now a peer dependency it might be a breaking change because you've to install it yourself now if you're not using npm >= v7. Apart from that all deprecations are removed now, meaning they are not throwing a warning anymore.
Check the migration notes when you upgrade
https://github.com/SassNinja/postcss-extract-media-query#migration