Removable lithium batteries like 18650 or 21700 are not really a thing (except for a few hobbies) because they're inherently dangerous and not a single respectable cell manufacturer sells them directly to end users (you can still buy them from unauthorized sellers though).
You can put your AA battery in the pocket with your keys and nothing scary will happen. Put there a 18650 cell and you might die from severe burns. So yeah, replaceable 18650 are awesome, but we'll never see them because people are dumb AF.
They're very common in flashlights, including mainstream brands based in the USA (Streamlight, Surefire) and in a bunch of stuff from the Alibaba to Amazon pipeline. The former uses cells with added protection circuits, and such models will accept generic third-party cells. I'm a bit surprised I don't hear about the latter exploding on a regular basis, but I have not.
Li-ion cells with protection circuits are safe enough for the average adult to handle without any special instruction; the risks are no different from the removable proprietary Li-ion battery packs that are common in power tools, and used to be common in laptops. There isn't a safety reason preventing their use in other electronics like Bluetooth speakers, though the business incentive to produce a more disposable product is obvious. New EU rules mandate user-replaceable batteries in the future, though I imagine manufacturers will find ways to make it proprietary and expensive if they can.
Untrue. I use them for 10+ years for anything that's possible (flashlights, powerbank and anything you can power with it) and they're not dangerous if you follow SIMPLE rules. About as dangerous as a bottle of vinegar. And you can perfectly buy good cells for any application, Sony vtc5 or vtc6 are two examples, never buy cheap ones and NEVER buy from shady sellers. And btw: you shouldn't put any battery to your keys in your pocket, that is called "stooo-pid" and makes little black holes in your thighs.