I am tall enough that shrinking t-shirts is a constant annoyance! (though I have to admit I haven't ever tried the 'conditioner and water' trick, even though I've heard of it before). Low temperature washes and avoiding tumble dryers works. I've also noticed thicker material t-shirts seem to definitely shrink a lot less! Much thinner cottton t-shirts seem to shrink a lot more, my mental model is that there's less mat…
Now I just wash on cold and hang dry all my cotton shirts, tees and button-ups. Just use a folding drying rack as simple as this:
https://www.ikea.com/us/en/p/mulig-drying-rack-indoor-outdoo...
It's a little annoying to have to leave the rack out in the middle of some room to dry overnight, but zero shrinkage ever. The way it fit in the store is the way it still fits three years later.
And no, stretching with conditioner/shampoo doesn't work, because there's no easy way to stretch it the "right" way -- as you tug on spots at the neck and the waist to pull them apart, they stretch but in weird, inconsistent, lumpy ways. The final result just looks like you've had small kids trying to hang from different spots on your shirt and it's all out of shape. Maybe in theory if you had some kind of stretching system with long clamps or something it could work, but who has that? Doing it by hand, it's definitely not a solution.