We can add multiple subnets to a single vlan. There is a feasibility in the networking world.. but it adds more complexity to the network, making it more difficult to understand by new admins. But it is possible in real time.
We can add more than one subnet to a single Vlan in a scenario where the existing network gets exhausted if the subnet IP pool gets exhausted. Then we can take this concept of adding a new subnet to the same Vlan without adding additional Vlans
But it should be a static environment. In a DHCP running environment, adding multiple subnets to a single Vlan is not feasible
All the network and security devices won't support adding more than one subnet to an interface. As per my knowledge, cybersecurity products like FortiGate firewall support adding more than one subnet to a single interface