Looks like this is for travis CI's integration w/ slack:

https://docs.travis-ci.com/user/notifications/#configuring-slack-notifications

Within the docs, they recommend encrypting the token (which it doesn't appear to be within .travis.yml, but I can't say for sure) if it's stored within a public repo, as it is here. Of course placing it within a private config repo should also work.

(and of course, with public exposure, the token value should change, if possible)

it can be stored in .travis.yml as an encrypted value, it would look like this:

notifications:
  slack:
    rooms:
      - secure: "sdfusdhfsdofguhdfgubdsifgudfbgs3453durghssecurestringidsuag34522irueg="
    on_success: always

That very codebase has been archived according to https://phabricator.wikimedia.org/T204490, however the information also exists in other active (?) ones:

$:acko\> grep --include="*.yml" -r slack .
./WikibaseQualityExternalValidation/.travis.yml:  slack: wikidataquality:6RzuL6LCKzoPupOKuNUhpCSt
./WikibaseQuality/.travis.yml:  slack: wikidataquality:6RzuL6LCKzoPupOKuNUhpCSt
./WikibaseQualityConstraints/.travis.yml:  slack: wikidataquality:6RzuL6LCKzoPupOKuNUhpCSt

There's a GitHub bot in Slack that shows CI results for PRs, you don't need to touch the Travis config for notifications at all.

This must be left over from when the group of students originally developed the extension.
We don't use slack at WMDE, so we can probably just remove the token now.

Patch for WikibaseQualityConstraints https://gerrit.wikimedia.org/r/#/c/mediawiki/extensions/WikibaseQualityConstraints/+/481995/

Can we also remove it from archived extensions? I guess we can't write to those?

Yeah we can just remove the integration with Slack. It's a leftover from the students.