I suspect that at some point, all of today's cable vendors have switched from lead to a zinc-based "pot metal" alloy.

I have filed many of them down, and the rate of cut along with the coloration says zinc to me.

It might take a lot of heat to melt these ends, but can't hurt to apply a soldering tip, with perhaps a bit of flux and solder to transmit heat initially.
The heat may loosen the grip on the cable end even if it doesn't melt, and a penetrant can be applied while the parts are still warm.

I usually just use a penetrant before struggling further with stuck cable ends, as often the cable end is inside of an integrated shifter.