I’ve cleaned my Garrard rubbers for years in mild soapy water in a basin without any ill effects as I can feel or see. Now that I think about it, it is an organic detergent. I found that water alone didn’t clean fingerprints etc.
Edit: Just spent 5 minutes flopping the rubber and it's still as supple as ever.
Edit: Just spent 5 minutes flopping the rubber and it's still as supple as ever.