LED lights are usually between 5v (battery powered) and 12v (plug in the wall type). That’s why there’s a chunky power ‘brick’ built into the plug to step down from the supply in the house (240v) to the 12v needed to run the LED lights.
The old style fairy lights were incandescent bulbs, which is why they’d feel warm to touch (and could pop when they got too hot). Whilst LED lights are much cooler to the touch (the actual bulbs themselves), that brick step down block built into the plug can get hot, so I wouldn’t recommend leaving them on when you leave the house. It’s unlikely they’ll cause a fire (with modern RCD protection built into the supply/fuse board they’ll usually trip the ring/floor switch to cut the circuit) but I’ve had a few dodgy power blocks that get hot so wouldn’t want to risk it.
Also, why waste energy if there’s no one in to see them? Sure, LEDs are far cheaper to run & more efficient, but they still cost money to run.
We actually have all ours on wifi/remote access switches so can (if we can be arsed lol) turn them on remotely if we want, but I don’t think the cats are too bothered if the Christmas tree isn’t on!
The only drawback of newer lights are that you can’t have things like this that utilised the heating of the bulbs to make the solvent bubble!
I’d love some of these, but trying to find bulb replacements would probably send me round the bend!