Change 771960 had a related patch set uploaded (by Raimond Spekking; author: Raimond Spekking):

[translatewiki@master] [ImageSuggestions] Register extension

https://gerrit.wikimedia.org/r/771960

Change 771960 merged by jenkins-bot:

[translatewiki@master] [ImageSuggestions] Register extension

https://gerrit.wikimedia.org/r/771960

Revert with https://gerrit.wikimedia.org/r/c/translatewiki/+/771910 due to:

Error Output:
================
Cloning into '/resources/projects/mediawiki-extensions/ImageSuggestions'...
fatal: Remote branch master not found in upstream origin

translatewiki.net can work with branch master only. Please ping me once fixed. Thanks.

Sorry for the late response - I totally missed all updated on this ticket!
The default branch has been switched to master.

Unrelated: are there plans for also supporting "main"?
I suspect most new extension will end up being created there (given how many of them start out on GitHub, where "main" has become the default)
If not, it might be useful to be document the expectation of the default branch being "master" somewhere in the extension creation docs?

In T302415#7837407, @matthiasmullie wrote:

Sorry for the late response - I totally missed all updated on this ticket!
The default branch has been switched to master.

Works now. Deployed to translateiwki.

Unrelated: are there plans for also supporting "main"?
I suspect most new extension will end up being created there (given how many of them start out on GitHub, where "main" has become the default)
If not, it might be useful to be document the expectation of the default branch being "master" somewhere in the extension creation docs?

I don't know. Did not found a task on Phabricator about non-master.

I believe that the switch from master to main was planned to be done together with gitlab migration to cause least disruptions.