If you delete an issue, its attachments are killed off too, so yes, you will free up space.
I'd recommend not deleting the issues though, as they're gone for good. I'd recommend only deleting the attachment, and leaving a comment like "attachment x deleted to recover space", so you keep the history of the issue.
Yes it could be an error. It is more likely to be a caching problem though - I've had a couple of cases where people killed off a pile of projects on a Wednesday, got the email again the following Monday. But they didn't do anything about it, and the following Monday, no email. That suggested Atlassian are looking at disk usage every Tuesday and sending the reminders a week later. (Don't quote me on the days or that is what is happening, I don't know, just that the emails didn't stop instantly).
I'd give it a bit of time to catch up with your work and stop bothering you, but if you give it a couple of days from now and still get another one, then
if you're on a supported tier, I'd raise it with Atlassian as a support request.
If not, then reply here again, and I'll see if I can get Atlassian to look into it for you. (and chase up the request for "on free/starter accounts, a report listing the issues or projects chewing up all my space")
👋 Hi there Jira Community! A few months ago we shared with you plans around renaming epics in your company-managed projects. As part of these changes, we highlighted upcoming changes to epics on...
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