Hydrogen embrittlement is a fascinating subject that pops up from time to to time in my industry.
I'm not sure we know all we know about HE, but I doubt that it would be an issue with chrome plated carburetor slides. High-strength steels, such as those used in manufacturing socket head cap screws and ultra-high-strength bolts are definitely susceptible to hydrogen embrittlement. But materials such as brass, stainless steel, and aluminum are not so susceptible and, some say, immune to HE. And any reported failures in plated brass components can often be traced to other issues, such as stress risers owing to cracks in the plating. Chances are the carburetor slides are made from soft brass, which should not fracture under normal use. Notably, releasing the throttle suddenly from a full-open position, allowing the slide to "crash" down on the stop-screw, may cause some cast brass material to fracture, regardless of plating.