Extended solutions have not been implemented in their general form described in that blog post (i.e. as recursive structures). There're less disruptive extensions like array literals and path literals which, as Jess pointed out, are mostly for internal use on the server side. Some functions/constructs have been extended to deal with those, eg. if your query generates an array literal, you can
UNNEST it into a sequence of solutions or serialise to a meaningful string using the standard
The main reason we haven't done the whole thing was that in 2017 it was mostly perceived a necessary step towards integrating path queries into SPARQL. However eventually we did the Stored Query Service which allows running path queries within larger SPARQL queries, and without extended solutions (as they're described in the post). So far it covers a lot of uses case that have been reported to us.
There're other incremental tickets towards extended solutions, e.g. for
UNNESTing paths into a sequence of solutions each representing an edge, so the work is ongoing.