This will necessitate a corresponding change to the docs at toolkit/components/telemetry/docs/data/environment.rst

Last time I removed a field from the docs when I removed it from the ping, I was told not to...

Odd. The docs should reflect the current state of reporting, so a field no longer sent should no longer be in the docs (except in the Version History at the bottom of the page).

Do you remember when/where that was?

I may have just been thinking of https://bugzilla.mozilla.org/show_bug.cgi?id=1420908#c6 but I feel like it happened somewhere else too.

Ah, yes, the old "you can remove the data but changing the shape of it will cost extra". In this case the schema for Environment doesn't require that addons.persona remain (it's not in a required list).

It might be worth an email to fx-data-dev warning of its removal in case there's any ad hoc analyses using it (not that I imagine there are).

But back to the docs: I think it would be wise to remove it and include the removal notice in the Version History. If you don't feel comfortable with that, a comment "removed by bug 1525511 in Firefox 67" would probably do just as well.