I tried to setup a different project build (ProjectB) based on either branch update or build finished in other project (ProjectA).

I though that "Applicable Projects" would do just that but after reading a bit (https://code.onedev.io/onedev/server/~issues/1064, https://code.onedev.io/onedev/server/~issues/109) this is not the intent of the field.

I found one older issue: https://code.onedev.io/onedev/server/~issues/1837 that suggest using REST to trigger downstream build -- is it still the only option to do that (it is 2 years old).

Is there any other option to flip the triggering: start downstream build by specifying which finished jobs / changes in repository in upstream? That is specify in ProjectB that it should start build when ProjectA finished instead of naming ProjectB in ProjectA.