Given the chip architectural changes in the last 6 years, I'd hope to see at least more than one release in that time frame.

Often (not allways, naturally) it's enough to just rebuild with a newer compiler to leverage the new arch features.

Out of interest, what are some new things (those bringing real performance gain) from the last 6 years which a standard library needs to adapt to actively?

Edit: also note that OP made this pointed on purpose, the last release is from 2018, one year ago, not 2013...

The standard library benefits a great deal from ASM optimized implementations of functions. New instructions get added that can speed things up significantly.

the compiler will almost never automatically generate this code.

This is all true, but I’m not sure the authors of these libraries are that interested in performance.

It needs to keep up with underlying changes in the operating system from time to time.