What is the STDC_VERSION value for C11? asks about C11, and How can I use "nullptr" for null pointers prior to C23? says that the current versions of GCC and Clang define it to a placeholder value 202000L.
GCC 14 Release Series Changes, New Features, and Fixes has now added the std=c23 flag, but doesn't mention what __STDC_VERSION__ expands to.
From N3220:
__STDC_VERSION__ The integer constant 202311L
| popular name | edition | constant |
|---|---|---|
| C23 | fifth | 202311L |
| C17/18 | fourth | 201710L |
| C11 | third | 201112L |
| C99 | second | 199901L |
| C?? | first, amendment 1 | 199409L |
It was in the first edition, amendment 1, to C89/90 where the addition of the predefined __STDC_VERSION__ macro was made. I'm unsure if it goes by any popular name but C94/95 has been suggested.