Does a gas powered water boiler need to be decommissioned?

Yes is the safe answer. The redundant pipework carries a flamable residue after years of use. If the gas appliance is to remain in place all the open ends must be capped by a gas safe registered engineer who can test the capped pipework for tightness. If it is to be removed then again a gas safe registered engineer must disconnect and cap the gas pipework and open ends, flue, and remove, or in fact at that point anyone can legally remove and brick up or whatever the situation. In any case a gas engineer must de-commission the gas supply and make safe the redundant pipework or remove and cap back to the meter. This is to prevent any air/gas mix in the pipework becoming dangerous or leading to a potential explosive hazard. hope this helps :)