Here are some of the alternatives I found with some quick research:

• https://github.com/osTicket/osTicket
• https://github.com/zammad/zammad
• https://github.com/opensupports/opensupports
• https://github.com/bestpractical/rt
• https://github.com/eltrino/diamantedesk-application
• https://github.com/django-helpdesk/django-helpdesk
• https://github.com/Taskulu/deskulu

Thanks -- didn't notice that when I was looking on their website.

All of the forks are very new (apparently OTRS community edition was discontinued without very little warning). It would be risky to choose one now only to have the fork abandoned soon. As for the paid version, that seems to have its own problems because my understanding is that all data would be hosted on OTRS servers.