The amount of misinformation being promulgated here is astounding. By US law to be classified as bourbon it must be aged in new, charred oak aging barrels, thus all the old bourbon casks for scotch production.
As to Sherry casks if one actually goes to the distilleries you will find that it is against Scottish law to add ANYTHING other than the standard 3 ingredients. Sherry casks are cleaned multiple times both hot and cold bec they do not want any sherry in the scotch they are fined if there is any sherry in it. to their best understanding there is a chemical reaction in the wood after the sherry ages in it. which would not be a kashrus problem.
Most organizations take the black and white – sherry cask bad stance because it is what their constituents demand. they think they know everything but half the picture is not the whole story.A little knowledge is a dangerous thing