Is using a TLS/SSL termination reverse proxy currently possible in Drupal 8 I use Nginx (port 443) which terminates TLS/SSL then forwards the request to Varnish (port 80) and on to Apache (port 8080). This worked really well for D7.

Am I correct in thinking that because of the following bug this set-up cannot work without 'Mixed Content' errors due to CSS and JS being written as absolute paths rather than relative paths?

References to CSS, JS, and similar files should be relative to server: less bytes, avoids mixed content warnings

I am looking for conformation of this question to ensure that it is not my server configuration at fault.
The patch you reference was applied to Drupal 8 last month. Drupal 8.0.3 was released 3 February and included the fix. If you're using this version (or later) you should be fine.