I just received my new SP101 (.357 Mag). It too, was missing the disassembly pin. I contacted Ruger regarding this, and the reply was that as of April 2019 they are no longer including the disassembly pin with new revolvers. There is no mention of this in the materials that come with the revolver.
I will contact them further to suggest they probably should include a small insert to the manual letting purchasers know. I spent considerable time looking for something that was never there. But hey, at least my desk is now cleaner.