Is My Hero Academia: Illegals canon?
It is canon, according to MHA's English Translator.


  Since a lot of people probably aren't familiar, this is a spinoff series to My Hero Academia. It takes place 4+ years before the events of MHA (they say Izuku is in elementary school), and features cameos by Eraser Head, Midnight, Tsukauchi, Ingenium, etc.
  
  It's all canon.