The Sling Attic is a place for retired modules, that we do not expect to develope further.
The Attic is not here - this folder is just meant to make it more visible in our main code repository but the attic is at https://svn.apache.org/repos/asf/sling/attic/
Modules which are found there might be moved back to our main codebase if someone needs them and is willing to do the work. Please discuss any such cases on the dev@sling.a.o list.
https://cwiki.apache.org/confluence/pages/viewpage.action?pageId=65873121 has more information about a first batch of module retirements that happened in 2016.