Greater Anglia allow singles and returns on smartcards, but don't advertise it on their smart card pages.
I think they might be still ironing out some of the issues, for example I've had an error message when doing the return as the gates would just give an error message. When contacting customer services I was told to use other ticket types (mobile or paper) until the issue is resolved. I've not heard any more even so they said they be in touch when the issue was fixed.
I also had an issue with the wide gate at Ipswich being in flip flop mode where it seemed to get part of the way through the process of putting the ticket on the smartcard then flipped to the opposite direction and then erroring rather than letting me through. I've never liked that single wide flip flop gate.
I discovered single and return smartcards had been released when I was booking my normal lpswich to Cambridge North tickets.
The greater Anglia smartcard ticket info pages are very out of date at the moment.
Several of the Abellio operators allow you to use an Android phone to add the ticket to the smartcard. When I tried in July, a greater Anglia smartcard would not work with the ScotRail ticket machines give wrong your of smartcard error.
Edit: reason for checking the ScotRail machines is they also support singles and returns, even have some cheaper of peak only tickets only available on smartcard