As I mentioned, when the feature was released there was no 5G possibility.
We would be interested in having a 5G only option, but would also keep he 4G/LTE only option available.
The development team always has much to do and this is not currently high on the list of priorities as LTE/4G only already provides a reduced attack surface option.
Since GrapheneOS added that feature AOSP introduced the ability to disable 2G networking which provides another option to reduce cellular network attack surface for people who dont want to use LTE only option.