To toggle item frame visibility, sneak + right click with an empty hand.
Alongside being able to toggle item frame visibility, holding an item frame in your hand will make all hidden item frames visible.
Installing this mod only on the server lets you toggle item frame visibility.
Installing this mod only on the client lets you hold an item frame to see all hidden item frames. If the server does not also have the mod installed, then all item frames will be invisible.
Installing it on both gives you both features.
If you come across any issues, post an issue on the github repo and I'll get to it as soon as I can.