Summary:
If you leash entities in creative mode, you can then switch to spectator and still be holding the leashed entity

Steps to reproduce:

1. Switch into creative mode, summon a cow, and obtain a lead.
2. Attach the cow to the lead that you have.
3. Switch into spectator mode and as you do this, take note as to whether the lead is still attached to the cow or not.

Observed results:
Spectators can have leads attached to entities.

Expected results:
Spectators would not be able to have leads attached to entities.

Image: