I love this question because it flips one of the previous questions to focus more on the individual. IMO the success of a Technical Product/Program Manager largely lies in the ‘What’ and the ‘How’.
What impact did they have? This individual measure of success should be tied to the business (product or platform) goals. The TPM should directly be held accountable for delivering on those goals. This is also the (relatively) easy part to measure (Eg: How many new users signed up for the app?, How much incremental revenue did the feature bring?, Did the platform ship on time?“How did they land that impact? This is the ‘hard to measure’ part. For a TPM it is equally important to demonstrate leadership qualities such as - high EQ, deep Empathy, Conflict resolution, crisp communication, ability to influence without authority etc and overall be a kind and respectful individual. Often peer feedback has proven to be an effective means to gather these signals.
To me, success of my product managers is measured based on their understanding of the product, ability to deal with partners, customers and executives and influence others. The way I would measure the same is based on their vision and roadmap documents. The product intuition can be judged from these artifacts and the execution can be seen through their interactions with team members, partners and customers.
Most of the aforementioned are hard to determine or judge. An easier way to look at how they are able to successfully perform in their role is the depth of their work, understanding and data being their rationales. If a PM is able to make quick decisions and back it up with solid data, they are leading the product successfully.