After decades of Campagnolo ergolevers on road bikes, and Shimano (not so rapid-fire) indexed gearing for flat bars, I'm planning to ditch indexing on the bike, and return to friction derailleur gear changes: for the simplicity, durability, control, and resistance to manufacturers continually changing or phasing out their products.
Flat handlebars mean searching out second-hand or new thumb shifters. There's also the choice of using Paul or Velo Orange thumb shifter mounts & separate levers such as the Rivendell's Silver or Gran Comp ENE.
Shops such as Fresh Tripe, Spa and SJS Cycles, Velo Duo, and Velo Vitality* have been great for sourcing different, difficult to find, or unusual parts for bikes.
Likewise, our old steel frames with 135mm spacing, decent clearances (40mm tyres with mudguards), all external cabling, and standard bb, hopefully keep things easier for future changes/ obsolescence. Triple chainsets, 8-speed cassettes & derailleurs might not be around so long, but let's hope so.
* We don't have any links to these bike businesses, only as happy customers.