Server add-ons are not allowed on Cloud, except for the pre-selected list of ones Atlassian are willing to support there.
You can install Connect add-ons, but the structure for those is quite limiting and there's nothing really resembling a scripting add-on for them.
My mob include Jamie Echlin, the writer of Script Runner (for server), and he's looked at doing it on Cloud. I'm not going to pretend I understand half the limitations, but one example of why SR for JIRA Cloud has not been done is that there's no way for it to do Conditions or Validators on transitions, it can only do post-functions. The Cloud/Connect API simply doesn't expose enough power to allow for much scripting.
Hey Atlassian community, I help lead engineering at Sentry, an open-source error-tracking and monitoring tool that integrates with Jira. We started using Jira Software Cloud internally last year, a...
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