A database update causes the site to go down because Stable9 doesn't exist anymore

Hey sorry about this. Drupal core deprecating stable in favor of stable9 burnt us also. And we do keep building new sites and upgrading the distribution, without doing as much testing of our own existing sites as we should— we have started to try to isolate disruptive changes in a 2.x branch but had not done so for the switch from stable to stable9.

But… an update hooks starting with 9 should not run until Drupal 9, and stable9 definitely exists in Drupal 9 starting by 9.0.0: https://www.drupal.org/node/3107179 →

Are you on Drupal 10 and is the problem *stable* does not exist?

I believe the issue here is that the version of Octavia used, on the 8.x-1.x branch, has templates that extend templates from stable, and not stable9. As a result, uninstalling stable causes the site to break. I have removed the old hook and added a new one to the 8.x-1.x branch that reinstalls stable if it is not installed. This should fix the issue. Though I am not 100% sure, I assume that OP meant that "Stable does not exist anymore", and not "Stable9". I ran into this issue myself, and release 1.17 with update hook 9102 should fix this issue for any sites affected by the faulty update hook 9101. Very sorry to anyone to whom this caused minor or major inconvenience.

Oof as it turns out, my comment did not actually reflect the problem correctly. I finally ran into the issue of stable9 not having been installed, as it (stable9 and not stable) is actually a dependency of Bulma . So if stable9 is not installed, as OP suggested, there is a fatal error:

Drupal\Core\Theme\MissingThemeDependencyException: Base theme stable9 has not been installed. in Drupal\Core\Theme\ThemeInitialization->getActiveThemeByName() (line 115 of core/lib/Drupal/Core/Theme/ThemeInitialization.php). 

My previous fix only worked because the site I was testing on had both stable and stable9 installed. I thought the issue was with stable because there was ONE TEMPLATE in Octavia that extended stable. Apparently all other templates require stable9, and uninstalling stable is actually safe once this template extension is fixed. So the real fix does involve the installation of stable9.

Though I still can't say that I understand the OP's issue, since update hook 9101 should have done just that at the time this issue was written.

I can only guess that the original issue was that stable9 did not exist because this update occurred on a Drupal 8.9 instance. I have removed compatibility from 8.9 on both the 8.x-1.x and the 2.0.x branches, since both of these currently depend on stable9, which is not in Drupal 8.

Oops sorry didn't mean to revert your changes to title and version. Thanks for updating those.