At least in its current incarnation, the 5G core network specification in 3GPP TS 23.501 makes it quite clear that 3GPP aims to cut the cord to older network generations. Handovers, for example, have only been defined between 4G and 5G. No interworking is planned to 2G and 3G networks. But as in Asterix comics where all of Gaul is under control of the Romans except for a small village there is one exception to this rule: SMS!
Yes, you read right, the 5G core network and mobile devices will still support SMS transfer over NAS (Non-access stratum) messages. As shown in 23.501 Chapter 4.4.2, the traditional SMS service center that saw its first use in December 1992 is connected to the 5G core network via the SMS-Function (SMSF) and the N20 interface to the AMF (Access Management Function) which will send and receive SMS messages over NAS messages to and from the UE over the 5G RAN.
I guess it is not really surprising. Despite all the different IP messaging solutions today, SMS is still used for many services and I guess quite some people still use it for person to person messaging as well. Still, it makes me smile that after 30 years, there is still no end in sight for this revolutionary service!