I accessed the database for psychology literature (PsycInfo) at my university library, and did a search for adult autistic outcome. There were 826 results, some of them quite recent. I would say there is ongoing active research on the topic, it just may not be easily located or accessible by Google search.
For instance, this report appeared interesting: Mindfulness-based therapy in adults with an autism spectrum disorder: Do treatment effects last?
Authors: Kiep, Michelle, Adult Autism Center, GGzE, Eindhoven, Netherlands, [email protected]
Spek, Annelies A., Adult Autism Center, GGzE, Eindhoven, Netherlands
Hoeben, Lisette, Adult Autism Center, GGzE, Eindhoven, Netherlands
Address: Kiep, Michelle, Adult Autism Center, GGzE, P.O. Box 1418, Eindhoven, Netherlands, 5626 AB, [email protected]
Source: Mindfulness, Vol 6(3), Jun, 2015. pp. 637-644.
"In conclusion, MBT-AS appears to be an effective method for reducing a variety of symptoms, and treatment gains remain stable over the longer term. "
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A finger in every pie.