I haven't subscribed since the early 2000s. Could be interesting.
What everyone now thinks of at the normal (purist you said) barrel treatment of Scotch is really just an accident of economics. So is Sherry cask aging as well. Initially French oak casks were used as they were in wine, either new or those that held wine. Bourbon, by law, has to be aged in brand new charred oak barrels. So after prohibition the U.S. had a lot of barrels it couldn't use so the Scottish bought them because they were cheaper. Sherry casks
came into vogue much earlier. The English used to love Sherry, and every pub had a cask of Sherry behind the bar. Those empty casks were cheap to acquire, and Scotch distillers used those to age their sprits.
So it's likely that wine casks were first, then Sherry, then ex-bourbon is the most recent, part of a shift to lighter styles that is fairly recent. So, what would purists really consider the original? Not ex-bourbon. Note that most sherry cask Scotch now uses new barrels that hold a garbage version of Sherry that is never consumed, solely to season the cask, since wooden Sherry casks are no longer shipped overseas while full since 1981. Sherry casks today cost far more than ex-bourbon.
Bourbon, because it must be aged in New Charred oak, can only be "finished" in a wine, port or sherry cask and still be called bourbon. American malt can be aged in any wood cask really. Irish too, although they use ex-bourbon for the same reason the Scottish do.