Messages when running Mix with Vue 2.6: postcss.plugin was deprecated

I'm seeing this message when running Laravel Mix:

trim: postcss.plugin was deprecated. Migration guide:
https://evilmartians.com/chronicles/postcss-8-plugin-migration
add-id: postcss.plugin was deprecated. Migration guide:
https://evilmartians.com/chronicles/postcss-8-plugin-migration

I'm struggling to get the error to go away. I'm assuming this is being caused by a sub-dependency but can't seem to figure out where this is coming from. Merely adding postcss to the peerDependencies hasn't removed the message.

Has anyone else run into / solved this?