Non-Bitcoin Uses for Old ASIC Miners?
Anything that makes ASICs suitable for applications other than the Specific Application the Integrated Circuit was designed for would probably slow it down or make it more power hungry or expensive.

One side effect they do have (along with anything else electronic) is that they convert 100% of the electricity into heat. So you could use them instead of an electric heater.

Note that heat pumps are actually more efficient, so it's not optimal. And in a lot of countries burning natural gas for heating is cheaper too.