There's just as much versioning for Relay Modern (from the server's point of view) as Relay Classic: so long as you have a client that sends a query string with old fields, your server needs to maintain support for returning those fields. So long as the field is nullable, you always can choose to "turn it off" server-side by always returning null.
In general, over time, as applications get more complex, the main queries you use will have more fragments. But we usually convert an entire query into a single ID (representing the entire query text, fragments and all). With each "version" of the query, fragments could be added, removed, or completely modified. Usually, your server shouldn't care what the specific fragment version is, but should be able to translate the fields the fragment asks for into a function that builds a response of that shape.
But one of the advantages of GraphQL in general (Relay included) is that you don't really need to version your API: so long as a field continues to exist in your schema, and continues to return the same type of object (even if that object adds new fields or interfaces), your old query/fragment will always receive the same shape. That may mean, over time, you "turn off" more and more fields by having them return as nulls to every client that asks for them, but you shouldn't think of it in terms of versions.
In general, over time, as applications get more complex, the main queries you use will have more fragments. But we usually convert an entire query into a single ID (representing the entire query text, fragments and all). With each "version" of the query, fragments could be added, removed, or completely modified. Usually, your server shouldn't care what the specific fragment version is, but should be able to translate the fields the fragment asks for into a function that builds a response of that shape.
But one of the advantages of GraphQL in general (Relay included) is that you don't really need to version your API: so long as a field continues to exist in your schema, and continues to return the same type of object (even if that object adds new fields or interfaces), your old query/fragment will always receive the same shape. That may mean, over time, you "turn off" more and more fields by having them return as nulls to every client that asks for them, but you shouldn't think of it in terms of versions.