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.