What does the Perfmon counter "Suboptimal plans/sec" actually measure?
There are two items this counter tracks:


Timeout
Memory Limit Exceeded


The values can be found in the XML plan as Max has proposed in the comments. I can confirm it has been this way since at least SQL Server 2008.

Kudos to @Max Vernon and @Joe Obbish.

The MSDN BOL Technet Docs description isn't super helpful.