It looks like a regression also when ScotchBonnet is not enabled. It's still possible to add engine through the right click context menu, though oneoffs shouldn't have broken.
Bug 1933283 Comment 1 Edit History
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~It looks like a regression also when ScotchBonnet is not enabled.~ QA pointed out this is not true, so likely dupe of Bug 1932106? It's still possible to add engine through the right click context menu, though oneoffs shouldn't have broken.