Some GC implementations want to always know when an object is written to, even if the written value is a special constant. Checking special constants in rb_obj_written was a micro-optimization that made assumptions about the GC implementation.
ivptr
fields
next_iv_index
next_field_index
RBIMPL_ASSERT_TYPE
Object#object_id
main
ruby_thread_has_gvl_p
clock_gettime
clock_getres