A post from a Broadcom employee over in the Fusion community indicates that they have changed the software update infrastructure for their products. Along with this change they have discontinued the ability to use the in-product updaters for Fusion and Workstation.
You will need to download product installers from the Broadcom support portal in order to update your Workstation installation. This limits you to updating to Workstation 17.5.2 or later.
Note that the “hacks” that allowed access to older versions of Workstation by direct URLs to the software update infrastructure no longer work.
My take on this is that they have not decided when, if, or how they will return in-product updates to Workstation. If they do, I would expect that it will need a product update to re-enable it.
It’s not great that they released this change to their users with no notice.