It is still possible,
With this repo by @jarlissonlira2 : GitHub - jarlisson2/appinventor-sources at edi1
I am able to generate my BackgroundTasks extension with service element in it,
Now you would have a question in mind that how can it work without update
It’s because main extension information is stored in 4 Json Files inside extension in which 2 of them contains all the annotation and if you add service there then it will work(As a activity inside the json)(Not in extension)
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