I posted the below on the Drush git, but I got redirected here. So here we go :).
With the recent core update I installed drush8 on all of my websites and updated 24 cores, both D7 and D8, using drush 8.1.16. As Drupal seems to release new cores more frequently than before (and since I was around for Drupalgeddon1) this was the first time using Drush on all of them to save time. But I did read the instructions, apparently I should have used Drush9 for Drupal8 updates.
First question: can existing Drupal8 websites be updated using Drush8? It seems I didn't run into any problems doing so. But then why does the drush site not mention D8.3+ is supported by drush 8? On drupal.org I found:
if you are a long-time Drush user, you may still use Drush 8 (8.14 or newer) to update Drupal 8
This information is either correct, in which case it should be pointed out on the drush docs that using drush8 is fine, or it isn't in which case Drupal has to update this section. When reading the following (they use dev8.5 in the example) I'm assuming these are all drush8 compatible commands: https://www.drupal.org/docs/8/update/update-core-via-drush I also checked the last commits to drush8 which seem to mention uodates for the D8.5 release.
Second question: how do I use drush9 on an existing website? Actually, I did look this one up and the process seems very cumbersome. Most of my D8 websites are quite small and from what I understand it's best to start from scratch again... What I found most usefull in drush was that it allows you to make a complete code dump and it stores updated files in the drush backups directory. So let me rephrase the question: Will drush9 and composer ever reintroduce the backup functionality in the future? The docs state: archive-dump has unfortunately been removed from Drush 9, so you'll have to use a standard CLI tool to backup the needed directories - typically excluding the vendor directory. Why did this functionality not make it into 9? Is there a way to specify which drush composer should use while keeping drush8 global for backup-purposes?
Story of my life (I didn't write it, but some will relate) can be found here: https://www.drupal.org/forum/support/post-installation/2017-03-24/how-do-i-migrate-existing-d8-site-into-a-composer-managed#comment-12328927
Thanks to anyone able to help me and many others keeping their websites safe (the easy way ;)).