A courtesy (small) clef at the end of a system announces the next system's new clef

When the clef is about to change at the start of the NEXT system, a small courtesy clef (in the new clef, at reduced size, matching the change-clef convention above) is printed at the very end of the current system, after the last note, giving the reader advance notice before the page/line turn.

A system ending in one clef with the next system silently opening in a different clef, with no advance courtesy clef at the end of the first system to prepare the reader.

Gould, Behind Bars, Clefs chapter, states the courtesy-clef-at-end-of-system convention explicitly, alongside the equivalent and more commonly known courtesy-accidental convention. Ross, The Art of Music Engraving, agrees this is standard professional practice, particularly valuable at a system break (as opposed to mid-line, where the reader has no turn-of-page/line disruption to prepare for). Read, Music Notation, agrees.

This rule needs the notated content and the rendered result together — the score says what should happen, the page says whether it did.