So I’ve been off the bike for a decade and the kids are old enough that I have something I thought was extinct, free time! I’m rebuilding one of my old bikes and I own lots of 9speed xt and xtr kit. I’d like to run an 11 - 42 cassette on a DT Swiss 240 rear hub, with a 34t narrow/wide chainring. Can I use my old 9 speed RD with a new SLX or XT 11 speed shifter? Technology has been rolling along in my absence, I’m still going to be using 26” wheels, 1 1/8th straight steerer forks, I should get on the retro bike forum 😀
I'm pretty sure the answer is 'No' but why not just get a 9 speed 11-42 cassette
https://www.ebay.co.uk/itm/387851246663
You'll probably also need a hanger extender
https://www.ebay.co.uk/itm/335801991220
Clearly you've been bitten by the bug in the past so run with what you have and wait for the new bike fever to start.
Thanks 👍