Why is the spam folder located under the inbox on Plesk Premium Mail? Can it be removed?

The spam folder was moved to INBOX.Spam in order to better integrate the extension with Plesk and other tools, like SpamAssassin.
In an attempt to remove INBOX.Spam folder via webmail, it will be restored immediately.

As a workaround, disable INBOX.Spam folder inside webmail settings:

Warning: Spam extensions like SpamAssassin won’t work with this workaround.

1. Log in to Plesk.
2. Go to Domains > example.com > Mail Accounts and log in to the required mail account.
   
3. Go to Settings > Folders > select INBOX.Spam and disable it:
   ​

mailbox Spam {
auto = create # autocreate Spam, but don't autosubscribe
special_use = Junk
}

mailbox INBOX.Spam {
auto = create # autocreate Spam, but don't autosubscribe
special_use = Junk
}