AFAIK, for a long they were pretty hostile and there's still this webpage which asks a $45 wire transfer in order to receive a CD with open source components used in the routerOS: https://mikrotik.com/downloadterms.html
That might have changed now, but I still can't find an official repo. You could look for older versions like this: https://github.com/robimarko/routeros-GPL or try to contact them directly (or sue if you think you have a standing).
That might have changed now, but I still can't find an official repo. You could look for older versions like this: https://github.com/robimarko/routeros-GPL or try to contact them directly (or sue if you think you have a standing).