After a few months of testing, I'm happy to announce Hosted Apache Solr now supports Search API Solr with Drupal 8! Both Search API and Search API Solr have been getting closer to stable releases, and more people have been requesting Drupal 8 search cores, so I decided to finish testing and updating support guides this weekend.
Note that getting Solr and search pages configured in Drupal 8 is a bit different than in Drupal 7 (even if you were using the Search API module instead of the older Apache Solr Search module)—I posted a detailed setup guide earlier this year: Set up a faceted Apache Solr search page on Drupal 8 with Search API Solr and Facets.
I still haven't migrated either this site or Hosted Apache Solr to Drupal 8 yet, so I don't have a canonical Drupal 8 reference site using Hosted Apache Solr at this time, but I've been working on a few other Drupal 8 sites, and am happy to report I can't imagine building a site in Drupal 7 again, if I can help it :) There are a few rough edges, but all the great new features more than make up for that!