in virtually all cases, "single-threaded performance" should be read as "serial performance" - the performance of a specific, single thread, within a larger program. While confusing to a layman, nobody uses the term to refer to performance when there is only a single thread running on the entire processor - we indeed left that time behind in the 90s, so that is not a sensible metric.

However, Amdahl's law means that serial/single-threaded performance remains an important determinant of total performance even in multi-threaded applications. Locking is a great example, even in a heavily multithreaded program you can be limited by lock contention, that specific part of the program is serial.

Gustafson's Law is of course a thing, eventually we will come up with additional work to fill those cores, but single-threaded performance remains a critical factor for CPU performance.