IMO the law is intended to reduce consumer electronic waste, making batteries that anyone from kids to grandparents can easily replace themselves. Currently the steam deck battery isn’t all that difficult to replace for the tech savvy “brave” folks who happily open up their devices and see the internals, but for most people that’s not good enough. I would never expect my 68 year old mother to replace it herself. IMO the aim is to make batteries replaceable like the PS5 SSD expansion capacity is. Just need a screwdriver and a few minutes, without every feeling like you might break something. Needing anything more than that and most people won’t do it out of fear they’ll break their device.
But only time will tell what actually happens.