Sites migrating from Cylc 7 to Cylc 8 will likely face a transition period with both versions running at the same time.
The built-in mechanism for inter-workflow triggering does not currently work across the 7/8 boundary, in either direction.
Cylc 8.1.0 will have a fix for triggering tasks off of Cylc 7 suites. We may also make another final Cylc 7 release for the opposite direction.
If you need this now (or soon) however, we can supply a small patch for your current installations.