Mohit Kumar in Intel Admits It Won’t Be Possible to Fix Spectre (V2) Flaw in Some Processors points to a microcode revision guide from Intel, PDF), which points to CPUs which won’t be patched for Spectre (variant 2) flaws.
Kumar lists the product families, Bloomfield, Clarksfield, Gulftown, Harpertown Xeon, Jasper Forest, Penryn, SoFIA 3GR, Wolfdale, and Yorkfield, but those are Intel names, not product names.
To simplify your searching for never-to-be-patched Intel chips, I created a list of the public chips names, some 192 of them.
Good hunting!