Any mechanic worth his stuff knows they have to readjust the chain slack after removing and installing the rear wheel. With the adjuster and plate missing, the part the long bolt attaches too, which goes around the axle, would have likely fallen out or slid back a ways as it is loose in the swingarm. If it was missing, he should have noticed immediately and talked to you about it. If it was there when you brought it, then he either forgot to reinstall it (not sure how you get away with that as the chain slack has to be set) and half assed the entire job, or didn't tighten the nuts so it fell off and he doesn't want to take responsibility for it. Don't trust that guy with your bike again.
Yes, I do have a few spares. It'll be a day or two before I can dig one up.
1990 EX500 - 142,000 miles as of August 12th, 2019
1993 Ducati 900SS - The real canyon carver