I have seen many networks in my years as a Network Engineer and none seem to use multi-area OSPF. I would guess only large SPs that do not use IS-IS run multi-area OSPF. Have you seen it in production before? Would you recommend it in production?
Do modern networks use multi-area OSPF?
Yes. Not all of them do (and many shouldn't), but some do.
I would guess only large SPs that do not use IS-IS run multi-area OSPF.
In my experience, this is far more likely in the corporate world. For example, I have seen individual WAN sites operated as OSPF stub areas or different OSPF areas within separate corporate campuses.
Have you seen it in production before?
Yes, but does it really matter? I have known network professionals who haven't run OSPF in any of the networks they managed.
What any given network professional has seen or not seen doesn't really make a difference to what makes good network design for a particular network.
Would you recommend it in production?
Absolutely, if the the network design called for it. Every network is different, so there may or may not be a need for it. Then again, there may be no need for OSPF at all.
Just because OSPF has been deployed on a network doesn't mean the person(s) who deployed it understood OSPF and as such it is not always deployed correctly for the situation. I have seen single area networks that should have used multiple areas. I have also seen multiple area OSPF deployments that made no sense and should have been run as a single area.