What the heck does this mean:

Backpack storage containers will no longer be registered in the query grid, to prevent some cases where plugins could find the containers and access them

How does it affect me?

Anything you don't understand in patch notes, you can generally ignore. I prefer to air on the side of accuracy rather than vagueness. Server owners who are impacted by this change likely won't know because only plugins which use the query grid could possibly be affected, and that's a technical detail not advertised on any plugin documentation. I'm expecting only plugin developers to understand that particular change note.

If you look at recent support threads, there is one where a server owner claimed that a bot plugin they were using was finding the backpack boxes below the map and looting them. This was theorized by the developer of that plugin as possible because that plugin uses an internal Rust system called the query grid to locate entities in an arbitrary location, which all entities are registered in by default. By ensuring that backpack boxes are not registered in that grid, the approach that plugin uses to find lootable boxes would not be able to find backpack boxes, therefore preventing the possibility that the bots were taking items from player backpacks. This was done just in case even though no concrete evidence was provided that the bots were actually looting backpacks.