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

instance method

Thread#[]=

thr[sym] = obj → obj

static VALUE
rb_thread_aset(VALUE self, VALUE id, VALUE val)
{
    return rb_thread_local_aset(self, rb_to_id(id), val);
}

Attribute Assignment—Sets or creates the value of a fiber-local variable, using either a symbol or a string.