Locked questions are supposed to be locked, that is they can't be voted, posted on, commented on or flagged. However, I noticed that while all these links are correctly absent from the normal view for a historically locked question, I can still see the flag link when viewing it from mobile.
Go to Fundamental difference between Hashing and Encryption algorithms from a mobile (or click the mobile link at the footer when viewing it), the flag link is visible and functional:
When I attempt to submit a flag, I get no feedback ("Thanks we'll look at it!" or "There was a problem"), and the flag isn't submitted (I don't see it in my flag history, and a mod doesn't see it there).
In addition, users with enough reputation can see the "edit tags" when hovering the tags:
When clicking "Save Tag Edits" it brings red error "Locked questions cannot be edited".