There are two axioms to keep in mind when packing for travel, especially if you're carrying everything on your back.
1. Everything weighs something.
2. Everything takes up space.
When I did my luggage purge back in Liddes and came away with 2 kg less in my backpack, one of the items I kept was a simple rubber drain plug. I think I paid a buck or two for it at a hardware store, but it was so long ago I really can't remember.
A shower is much more energy efficient than a bath, which is most likely why hoteliers never make a plug available even when there is a bathtub. Even sinks generally drain out immediately, which makes it challenging for someone hoping to shave, or in my case, do laundry.
That's why this simple little piece of rubber is invaluable for the road warrior or pilgrim. As long as it's large enough to completely cover the drain, the water pressure will hold it in place, allowing the weary aching pilgrim to dissolve the knots and pain in a hot bath. Just be careful not to bump it loose, or your relaxing soak may be interrupted!