I have a Velocity Dyad (26” symmetrical) and a Shimano XT hub (T780) that I’m about to buy spokes for after using the Sapim spoke calculator. The frame is a 1995 Ritchey Comp. Leonard Zinn doesn’t specifically say not to use a symmetrical rim to build a rear wheel. I don’t ride trails anymore, just county roads with potholes and gravel. Will I be wishing for asymmetry on my back end soon?
I have built MTB rear wheels with both asymmetric and symmetric rims. As long as you do a reasonable job and pick the right components, both will stand up well over time. I am not sure of the axle dimensions of that era MTB, but generally MTB have a wider rear axle which reduces the dish and thus the need for an asymmetric rim.
Asymmetric rims are at best a marginal gain. However the heavier the load and the lighter and narrower you're building your rear wheel, the more that marginal gain helps extend the life of the wheel. Dyads are already pretty durable rims, with the appropriate spoke choice and count they should build up to very reliable wheels.
Unless you have a history of breaking spokes on non-drive side of the back wheel, I would not focus much on asymmetric rims for a general purpose rear wheel build. They do make getting the dish correct during the build easier, ( especially for narrower road wheels), but like a lot of bike things they only matter when you are pushing the edge of performance.