As of the numerics reform in 8.5.0, nothing in the Tcl codebase calls strtod() anymore.
Tcl's configure.in still calls SC_BUGGY_STRTOD, though, and compat/fixtstrod.c is still present. This is presumably for the benefit of Tk, which also calls SC_BUGGY_STRTOD in configure.in, but does not include its own copy of compat/fixstrod.c.
It is not clear whether Tk will even successfully compile and link if the SC_BUGGY_STRTOD test triggers, or if the test is needed for Tk in the first place (looks to me like the answer is "no" in both cases.)
Confirmed: Tk does *not* successfully link if SC_BUGGY_STRTOD triggers.
Comments on the chat suggest this bug
manifests as "Tk will not build" which seems
higher prio to me.
Hmm... this is the Patches tracker, but no patch?