per oc set route-backends -h output:
Routes may have one or more optional backend services with weights controlling how much traffic flows to each service.
[...]
**If all weights are zero the route will not send traffic to any backends.**

this is not the case anymore for a route with a single backend.

at least from OCP 4.12 onward

all the time

1. kubectl create -f example/
2. kubectl patch route example -p '{"spec":{"to": {"weight": 0}}}' --type merge
3. curl http://localhost -H "Host: example.local" 

curl succeeds

curl fails

https://access.redhat.com/support/cases/#/case/03567697