How to unsplit List(text) paragraph fields while keeping data?

I thought you can add values on List(text) fields but not remove them.
If not via a config import then maybe you can do it with an update hook.

But I am afraid that config split can not do anything for you here.
I would try adding all the options to the field storage, and as long as the uuid is the same it should be treated as the same field and possibly update it on an import. But I don't know if that helps. If when splitting you had different uuids for the fields then you need an update hook to set them to be the same so that the config import doesn't remove and add the field again.