My take: Sophos put 25 million euro on the table to buy HMP and HMPA, so they are going to incorporate this in their flagship product. It is normal a specialized solution being bought by a general (all-in-1) solution product to be phased out or remain a test ground to iron out compatibility issues before implementing this in their flagship product.
Ideally you only want educated users to help you iron out all the bugs, not spend support on amateurs using your software in the wrong way. So it is not surprising they are removing the user documentation. So when you are already a user of HPMA, I would not mind that the user documentation is removed. The lack of user documentation is a subtle hint for people considering this software, stay away it is an insiders tool,
Just found the newspaper article regarding the takeover of HitmanPro by Sophos (link), it was not 25 million but 29 million euro. It is somehow a feat to remain an independant brand considering the takeover dates back to 2015. HMPA is only available as consumer product, for corporate/business use Sophos X is offered instead (link).
In the past you could get a free HPMA (testing) license when contacting the developers. Don't know whether this still is possible.