Where can I see the content of tickets I raised on the Stack Overflow community support portal (https://meta.stackexchange.com/contact)?
Generally, nowhere. We don't provide any utility for viewing your past submissions anywhere on our sites. We don't keep copies of the ticket submission anywhere in our own databases - only the fact that the form was submitted.
However, we do use Freshdesk, which has a separate portal with ticket viewing capabilities. We don't explicitly support this, which is why the login options are hidden in the portal, but if you can manage to get yourself logged in using the same email address you used to contact us, you can view your correspondence for the past year there (we delete tickets out of Freshdesk after they've been closed for a year). Last I checked, only the Google option allowed you to login to the portal without being a support agent - but I haven't checked in a while.
They are deleted because of GDPR. They contain private information that we don't wanna keep sitting around indefinitely, and it's too difficult to incorporate it into the process for when we receive requests via the form, so we just make sure they're eventually deleted always.