it looks like you are I-485 pending and ready to forsake your H1 status. Your old company (I assume also the I-140 sponsor) is doing you a favor, because they could just write USCIS to revoke your H1 and I-140, which will get your I-485 denied if you cannot find AC21 portable job in time. If your I-140 is NIW or EB1b, or LC based I-140 but I-140 approved and you have found a same/similar job with i-485 pending > 180 days, then you don't need to care about your old company, otherwise, better not anger them. bacsue they still can get your I-140 revoked and cause you a lot of trouble.