SHACL AF was initially published as the W3C Note. W3C RDF Shapes Working Group (WG) completed its charter in June 2017. SHACL AF was published as a W3C Note because the WG ran out of time to complete it and to take it all the way to Recommendation during the chartered period which is typically 2.5 years.
If there is enough interest among the W3C membership, W3C can charter a new WG and/or a new Interest Group. A WG can produce and publish W3C Recommendation. Both, Working Group and Interest Groups can publish W3C Notes.
So, to answer your question, W3C may begin work on a Working Group or Interest Group charter at any time, provided there is enough interest. The charter may be about publishing a new version of W3C Note or talking a Note all the way through Recommendation. Often, a starting point is W3C workshop. Any party (not just W3C member) can participate in a workshop. The workshop establishes the need and then a charter is developed.
Feel free to contact the W3C Team to find out the level of interest in another iteration of the SHACL specifications and/or to propose the formation of a group.