So have I. I did some research on them and they are proofed at over 300 psi. Made of mild steel about 1 mm thick. Depressurise by attaching the torch and leave on outside for a bit! But still gas inside. I bought a kit years ago of a torch with an oxygen bottle too, which lost most of its puff years ago, so I've adapted this as a small pressure reservoir, only for 40 psi. I'd be chary about doing the same with a gas bottle as getting all the propane out would be tricky.
Years ago I designed a gas bottle meter and that project involved cutting a calor gas bottle in half. I didn't do it myself but no one got blown up. If you did it outside on an empty cylinder with a hacksaw and plenty of coolant it would probably be safe enough. But as part of that project we found that gas bottles were, in the words of a university metallurgist we consulted, a "metallurgical dog's breakfast' so I wouldn't have high hopes of using them for anything else.
I gather that they can be recycled via the supplier, in principle, though whether any of the suppliers know that I doubt!