The patch/code needs to be rewritten. It's using really old code such as
db_select()
which doesn't exist anymore.It also needs to be converted to a Merge Request.
I will also note that this functionality can be done with the Flag module. If you hover over Drupal.org's subscribe link, it's using a flag endpoint, such as https://www.drupal.org/flag/flag/project_issue_follow/3021537?destinatio... → ...