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).
Which is Linux based, yet Mikrotik seems somewhat reluctant to honor the spirit of the GPL...
Supposedly its available by request (which is all that is required by GPL). An old-ish copy is here: https://github.com/robimarko/routeros-GPL
It seems they comply https://github.com/robimarko/routeros-GPL