Yeah, the modern 1.1mm shift cable rather suck for durability compared to 1.5mm, at least with vintage DT and thumb shifters from SunTour. I've resorted to using a short section of Jagwire "shift cable inner housing" over any areas of contact, notably through the BB and right around the shifter cable grooves. I can't say it really helps of not for fraying at the shifter, but it did stop the fraying under the BB since the cable guides are part of the BB shell, no nylon attachment guide. The inner shift housing is just like the inner brake housing except it's for 1.1mm cables. The larger for-brake stuff didn't fit through my frames cable guides.

Also, 1.6mm brake cables are too big to glide in Jagwire lined brake cable housing. I was going to use some new old stock brake cable on a new setup and I found out that way.