With the OVS mechanism driver, it's allowed to have multiple segments on the same compute for the same physnet. It's been developed and merged last year. 

Customer requests the support for this feature with ML2-OVN. 

The unavailability of this feature forces the cloud operators to create a new network whenever one of the subnets is full (for a given physnet). Then, users have to change their pipelines, IaC, and configuration to use the new network with the free IP addresses in the created subnets. Users cannot stack subnets in the same network which increases complexity and the management of the OpenStack platform.

Following is the corresponding spec for ML2-OVS upstream

https://review.opendev.org/c/openstack/neutron-specs/+/823823