^^^^^^^^ There, @Sooty25 has the nub of the whole debate.
I might have read about a Rotax installation, but the thread went into all sorts of arcane theorising about CHT sensors altering the depth of plug entry and some respondents advocated removing the plug-washer.
Rotax use (almost) standard spark-plugs. I say "almost", because the little screw-terminal on top is replaced by a 1-piece terminal-post of exactly the same profile......a good excuse to fleece the victim of another couple of quid, with the specious argument that the plug-cap is such a poor fit it will allow an ordinary terminal to unscrew and the cap will fall-off with the terminal inside it,
Apparently many owners substitute a different type of plug (Iridium, IIRC) which is of surface-discharge construction and much less prone to fouling.
I theorise that you can partiallly remove a screw-terminal and put a crimp in with pincers...that will make the threads nip. alternatively, just nip the thing tight with a pair of pliers.....for the truly anal, a spot of loctite secures the terminal.
Now, the interesting issue to this marketing "scam"....NGK apparently refuse to sell the removeable-terminal plug for Aviation use, in spite of Rotax having sold the engines with "not for aviation use" plastered all over! So, you go to a motorcycle dealer and get the cheaper one!
To get back to the core question. Anyone who has worked with copper, including plumbers and electricians) knows it work-hardens. but a single twisting of strands of wire, does not render it hard. likewise, logic dictates (remember, both the cylinder-head and the plug have machined faces) that a washer can be tightened several times before work- hardening makes deformation difficult.....but a gas-tight seal will ensue before that stage is reached anyway.
Flexy brake-hoses are fitted with solid copper washers, reused them if not damaged, never annealed, never had a leaker and never had to overtighten to the point where threads start distorting
(I cringe when tyre fitters use a "buzz-gun," changing wheels and leaving customers with stretched wheel bolts that won't budge when they get a flat tyre next.@Flying_john .....Presuming the incoming gas-charge flows across the plug-electrodes in the same orientation, then a side-electrode directly in the path will tend
to split the flow away from the spark-gap....otoh, if 2 of a triple-electrode design are either side of the gas-flow, they could be held to channel the charge in to the spark-gap......but, of course it's all bunkum because the combustion chamber is designed to make a turbulent charge for thorough mixing and an even burn. Likewise, a few thou different depth of a CHT sensor-washer, or a re-used plug washer is probably less than the difference in placement that worn electrodes will make to the plug-gap.
Marketing scaremongering at it's finest