Whether or not the OpenSearch autolink is included in XMLUI pages is controlled by the websvc.opensearch.autolink setting. If that setting is true (the default), the OpenSearch link is included even when OpenSearch is disabled (websvc.opensearch.enable = false, the default).
Browsers (eg Firefox) offer users to add a custom search engine for the repository based on the autodiscovered OpenSearch description. When the user attempts to add the search engine while OpenSearch is disabled, the user will see an error message ("Firefox could not download the search plugin from: [URL to repo OpenSearch description]").
Presumably this can be fixed by changing the logic here:
to only include the autolink information when OpenSearch and autolinking are both enabled.