Are any bikes still being made with cottered cranks? You can still buy the cranks, but I doubt that any new bikes come with them. I thought that center pull brakes were gone, but I saw the other day that you can buy brand new DiaCompe CPs. Also Paul CPs. Recently became the owner of a Raleigh Sports so had cause to look for 590mm/650A/26X1-3/8 rims and tires. You guessed it, very nice polished CR18 rims and a wide selection of tires including some real nice Schwalbes. Tires for 597mm 26 X 1-3/8 1-1/4 Schwinn S5/S6 rims are slim pickings (Kenda). Found a European shop on the web that had some inexpensive Schwalbes in that size, but shipping was like $150 so I passed.

Oh -- how about zinc plated carbon steel spokes. I bet they're damned close to being obsolete and unavailable. Handlebars wioth 15/16" clamp area -- can you still find those? Ashtabula cranks?

edit: various standards are gone, of course. 26tpi headsets (old Raleigh). 26tpi BBs (Raleigh again) though there may be som Phil Wood stuff. Swiss BBs. Italian threaded freewheels -- the BSC works on Italian threaded hubs, in my experience.