Possibly a Portugese idiom, and idioms don't translate well. I took it to mean sailboats designed largely by rule of thumb, lofted with battens, and styled after certain traditional uses and aesthetics. Might be easier to stipulate what isn't "old shoe": computer-designed hull forms with fine entries, no overhangs; wide beam carried all the way aft; crisp chines & flat bottoms for planing; deep, high-aspect keel with bulb; wheel steering, preferably two; bright, low-fuss interiors; more electronics than a video arcade; high-aspect, roachy, full-battened main; sprit or prod for reaching headsail & asym; tech-fabric sails throughout.
A Jeanneau 42
is not an old shoe. Looks more like a blindingly-high-tech basketball or tennis shoe endorsed by billionaire athletes. Which is fine if you play that sport, or if you want to impress other people. I stand on a concrete floor all day, so all I care about is something with solid stitching, hard-wearing leather uppers, good traction, and arch support. If I can find that at KMart prices, all the better.
Old shoes probably look like herring boats: long overhangs, bluff bows, lots of sheer, boxy or bubble coach roofs, low volume, tumblehome and pinched sterns. No carbon, short masts, skeg-hung (or God help us, keel-hung) rudders. Hanked-on headsails, tiller steering, a distinct lack of cup holders. You may have to stoop belowdecks, and you may need to look at shroud telltales or a masthead fly because someone failed to provide you with digital apparent-wind dials.
I'm cool with all that. As others have said: old shoes fit the purpose, and I have better things to do with my money than plonk $200 on a pair of Zoom Kobe IIIs
, ya know? They look like they were made by Beneteau.