Should I puncture my empty gas canister before disposal?

Generally you do not need to puncture your empty gas canister before disposal. In Europe (except Austria), you can put your empty gas canisters straight into the domestic recycling. You do not need to puncture them first.

Waste disposal and recycling organisations treat empty gas canisters like any other pressure canister (e.g. aerosols). After collection from recycling bins, canisters are are crushed, shredded, magnetically separated, melted down, and new recycled steel ingots produced.

Before disposal into recycling bins, make sure the canister is empty. You can do this by running it with your stove until the flame goes out or until empty.

There is a good summary of other aspects of gas canisters on the AlpineTreck / Bergfreunde website here: https://www.alpinetrek.co.uk/base-camp/transport-and-dispose-gas-canisters/comment-page-1/#comment-4970