Unmanaged packages: As a best practice, install an unmanaged package only if the org used to upload the package still exists. If that org is deleted, you may not be able to install the unmanaged package. What does this exactly mean ?
You can create unmanaged packages in any org, including Scratch Orgs, Sandboxes, Developer Editions, etc. Developer Edition and Sandboxes will be recycled after one year of non-use, and Scratch Orgs will be recycled no later than 30 days after creation (assuming the maximum time is used). If an org is recycled/deleted, then the packages created there will no longer function, as they will also be deleted.