Hi, and welcome to this Apple (user-to-user) Community :-)
Re: "Assistive Access: Links are not working in any app : Is that an intended restriction?
If so, where can I read about that in an official document from Apple."
"I configured assistive access for my elderly mother. She receives Mails, iMessages and Whatsapp messages. Not in any of those apps tapping a link has any effect."
Wonderful to know your mother has your kind and thoughtful help!
Am no expert in this area, yet your question certainly deserves some response.
We do not know which hardware model or software version your mother uses for Assistive Access. Eg: Linked articles below are from Apple Support for an iPhone with iOS18 or later installed: (If it helps: iOS version can be changed at top of articles for accurate info.)
Seems for any / all links to open, your mother would need unrestricted access to the internet. Since Assistive Access is designed for those with cognitive issues, Apps designed for assistive Access work differently to normal apps. For more info., see: Understand certain differences in Assistive Access on iPhone - Apple Support
Apps in Assistive Access can be customized, and many other apps can be added in their original form, yet that often means leaving Assistive Access to access Settings:
If needed: How to: Enter and exit Assistive Access on iPhone - Apple Support
How to: Change Assistive Access settings on iPhone - Apple Support
How to: Add, remove, and customize apps for Assistive Access on iPhone - Apple Support
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You can access much of Apple Support's info. for Assisttive Access online:
Eg: User guide for iPhone: Assistive Access User Guide - Apple Support
or
For iPad: Assistive Access User Guide - Apple Support________
If needed:
Expert assistance with Apple's Accessibility features can be obtained from Apple's Accessibility Support Team: (You can "Get Support" from the link at the bottom of their website:)
Accessibility Support - Official Apple Support
or
See: How to contact Apple for help with accessibility features - Apple Support
All the best :-)