Both Layer 2 and Layer 3 support vSAN connectivity between data nodes. Layer 2 does not require a static route, but Layer 3 does.
Using Layer 3 is recommended for connectivity between the data sites and the witness. Witness Traffic Separation (WTS) can be used with Stretched Cluster configurations under the following conditions:
MTU can be different. For instance, MTU of 9000 between data nodes, MTU of 1500 between data nodes and the witness.