This apparently was missed when support for Interix was removed in 2016.
gcc:
PR target/69374
* doc/install.texi (Specific): Remove stale reference to Interix.
@subheading Other Windows Platforms
GCC no longer supports Windows NT on the Alpha or PowerPC.
-GCC no longer supports the Windows POSIX subsystem. However, it does
-support the Interix subsystem. See above.
+GCC no longer supports the Windows POSIX subsystem.
Old target names including *-*-winnt and *-*-windowsnt are no longer used.