LinearLayout flagged if used in CardView

Apart from other suggestions and the above answer, this issue happenes on me if I didn't upgrade my support library to AndroidX, in this case is CardView. So from

android.support.v7.widget.CardView

will need to be change to

androidx.cardview.widget.CardView

and the warning shall now disappears.

Just to have a proper answer, as @Blacklight said in comments, it is an IDE issue so restarting it should fix the problem.