I would personally like this (which is probably obvious based on the previous Issues I've raised).

So would I, even to be able to release/resume a specific branch while the rest stays paused/held

Allow trigger-now in a paused workflow?

This use case has always been possible at Cylc 8, use the "hold after cycle point". Documenting this in cylc/cylc-doc#739

So would I, even to be able to release/resume a specific branch while the rest stays paused/held

This will not be possible via play/pause because this is a global setting (i.e. you can pause a workflow, but you can't pause a subset of it). However, this is possible via hold/release (which are local settings).

I.E. this is a hold/release use case, not a play/pause use case.

I think we probably intended cylc hold for this sort of thing

Yes.

Task hold and workflow pause are orthogonal features, there are advantages to being able to slam the brakes on a workflow, manual triggers and all. There are also interventions beyond trigger that can cause tasks to submit (e.g. set).

Holding all tasks in a workflow is a little clumsy at the moment due to future matching requirements (#5827), however, this is a priority issue to solve so should go away before too long. Holding all tasks via this interface achieves the desired result, IMO this is a better solution to changing the meaning of pause to satisfy this use case.

What remains (after #5827) is to improve access to task hold to make it easier to reach and more intuitive to use, ideas:

• Consider reinstating the cylc run --hold argument to start a workflow with all tasks held.
• Improve access to the Hold/Release commands in the GUI menu.
• Consider changing the default task list in the GraphQL mutations for Hold/Release from null to */*. This would mean that if you clicked the Hold command from the workflow menu, it would hold all tasks by default without the user having to type */* into the box.
• Fix this small data store issue data store: n>0 tasks past the hold point are not marked as held #6140 which makes the effect of "hold after cycle point" less obvious.

With this using hold/release would be as convenient as using play/pause.

there are advantages to being able to slam the brakes on a workflow, manual triggers and all.

Not sure I buy this. What's the advantage of being able to slam the brakes on manual "trigger now" exactly? By definition that means run this task now, so I wouldn't do it in the first place if there was any reason I didn't actually want it to trigger.

There's nothing fundamentally wrong with cylc pause meaning "the scheduler will not automatically submit any tasks" but the user can still do it manually.

It's conceptually (and implementation-ally) simpler than cylc hold "*" where users may have to think about exactly what the glob is going to match.

If we do make cylc hold "*" completely task-pool agnostic it would not matter that a similar thing might be achievable both ways in the future. After all, there's almost complete overlap between the effect of cylc pause <workflow> and cylc hold <all tasks>.