Google’s message reads:
Google has detected that your site is currently running PageSpeed < 126.96.36.199 or < 188.8.131.52, an older version of PageSpeed. Outdated or unpatched software can be vulnerable to hacking and malware exploits that harm potential visitors to your site. Therefore, we suggest you update the software on your site as soon as possible.
Following are one or more example URLs where we found pages that have outdated software. The list is not exhaustive.
This is similar to the notification Google sends out about updating your WordPress install or other old CMS program.