Mitel seems to have cleaned house. I find the download page in KMS but the actual download link is dead.
Go to KMS, Choose “Answers” in the top menu and then “Release information” below that.
Then Expand the menu tree on the left: “Contact Center Solutions” and first expand then click “MiContact Center Solidus”. This will give you releases from when the product was Solidus. I had to go back three pages in the list to find https://mitel.custhelp.com/app/answers/answer_view/a_id/1002812 which claims to be the download link for Solidus 8.3SP3 – but the download link given is dead.