End of Life Ruby 3.0 has reached end of life and gets no more fixes. Switch to Ruby 4.0 →

class method

Thread.DEBUG

Thread.DEBUG → num

static VALUE
rb_thread_s_debug(VALUE _)
{
    return INT2NUM(rb_thread_debug_enabled);
}

Returns the thread debug level. Available only if compiled with THREAD_DEBUG=-1.

See also .DEBUG= .current .abort_on_exception= .exit also in 3.1