It changed in 2024.4 https://rc.home-assistant.io/changelogs/core-2024.4
This is the relavent pull request
https://github.com/home-assistant/core/pull/113516
I’m not sure, as it isn’t applicable to my config, but I imagine you could just enter a fake entity following the guide at https://home-assistant.io/integrations/modbus For example a switch with a random address - just make sure it won’t interfere with your actual device.
modbus:
- type: tcp
host: IP_ADDRESS
port: PORT
switches:
- name: DummySwitch
address: 13
write_type: coil