The latest Microcode Revision Guidance Guide update from Intel reveals the chip giant ceased the development of Spectre mitigation techniques for some of its older processors. A month ago, Intel's guide indicated the company was "Planning" or had "Pre-Beta" code for some of its older CPU families, but now the list is showing a "Stopped" state.
This includes Penryn, Wolfdale, Bloomfield, and Yorkfield processors.
Overall, this is understandable as most of these chips were first introduced 10-11 years ago and went out of production in 2011.
The reasons for this "Stopped" state, as Intel puts it, are that "After a comprehensive investigation of the microarchitectures and microcode capabilities for these products, Intel has determined to not release microcode updates for these products for one or more reasons including, but not limited to the following: a) Micro - architectural characteristics that preclude a practical implementation of features mitigating Variant 2 (CVE - 2017 - 5715 ); b) Limited Commercially Available System Software support; c) Based on customer inputs, most of these products are implemented as "closed systems" and therefore are expected to have a lower likelihood of exposure to these vulnerabilities."