It's maybe library-specific rather than an industry term of art, but detached does have an existing meaning in Grand Central Dispatch which is similar to the meaning proposed for Swift, which is to dissociate the work item's attributes from the current execution context.
Yes, detach has a long history predating GCD with at least a decade+, used in at least e.g. Solaris threads with THR_DETACHED (1992) and in pthreads with pthread_detach and the PTHREAD_CREATE_DETACHED thread creation attribute (1995).
So I would say it's an established concurrency term/concept since quite a while back.