Microsoft runs FRR on SONiC.
Vyos runs FRR.
6wind runs FRR.
Cumulus Networks runs FRR.
Juniper runs FRR in certain products.
VMware runs FRR.
Broadcom is integrating it.
I don't think you are very familiar with the scope of changes that have gone in since Quagga. Not to detract from BIRD - which is a great, solid BGP implementation - but it is disingenuous to say FRR isn't used in production.
