Microsoft will soon remove Windows 10 free upgrade for assistive technologies because of misuse of the tool. The company clarifies that assistive technologies are for those with disabilities and who cannot use a computer in a conventional manner. In next couple of weeks, Microsoft will remove that web page and tool. If you want to upgrade to assistive technologies you require to contact MS support directly to upgrade.
According to Microsoft:
Assistive technologies are for those with disabilities and who cannot use a computer in a conventional manner (e.g. narrators for the blind, eye tracking machines for those unable to move a mouse, suck and puff machines, etc). Unless you use those, please do not use the link floating about to late upgrade. It is only for those with disabilities and is being changed soon to prevent abuse of the system.