Nowadays the WindowGlobalTargetActor is meant to debug only one precise WindowGlobal, We enabled "EveryFrameTarget" via a preference to true for a while now.
And we are dropping the related preference in bug 1741927.

Now, as highlighted in bug 1741927 comment 3, there is still ways to spawn a WindowGlobalTargetActor that debugs same-process iframe.
The targets created from Descriptor.getTarget() are still having the WindowGlobalTargetActor.ignoreSubFrames set to false:
https://searchfox.org/mozilla-central/rev/3269d4c928ef0d8310c2f57634e9b6057aa636e9/devtools/server/startup/frame.js#114-117
https://searchfox.org/mozilla-central/rev/3269d4c928ef0d8310c2f57634e9b6057aa636e9/devtools/server/actors/descriptors/tab.js#132-169
But this codepath is no longer used by any DevTools feature except may be the non-multiprocess toolbox? Everything else is now using the Watcher actor and have they target created by the Watcher and JSWindowActor.

So that we might have to coordinate with VS.code before removing this?
And we may also have to wait for the full enabling of the multiprocess browser toolbox.

About the browser toolbox, it looks like iframe debugging in the parent process seems already somehow broken... I can't get the iframe dropdown to show up. So may be we could move forward as we wouldn't break things much more.