I continued to use a hosepipe to wash both my bike and m/c during the last ‘hosepipe’ ban – all legal.
But not necessarily ethical
IMO water isn’t ‘scarce’; my local water company just hasn’t seen fit to either build more reservoirs or an infrastructure to bring it from areas where its available.
And we’ve seen the lowest rainfall for a loooong time. Reservoirs can’t store water that’s not there in the first place.
There’s a finite amount of water, and it’s not very much, available for use. Yes, there are valid arguments that that amount could be larger, but there’s nothing that can be done about that in the near future. So it is scarce, and using it for non-essential stuff like cleaning bikes strikes me as, basically, incredibly selfish.