I don’t think you have any direct way to make it hidden from Link Account attributes unless you don’t have it in schema
Can you please provide the actual use case of why do you need this in schema and why do you want to hide even after having in schema so that any alterative approach or work around can be suggested.
The use case is such that the date of birth is a protected attribute for my client due to GDPR data protection regulations, and not everyone should have access to it. In this case, individuals working in the IT department who can log into IIQ should not be able to see the date of birth attribute for any accounts when they enter the application.
If they are not admins, go ahead with the approach of Encrypted value during aggregation as @enistri_devo suggested.
But again if you need admins to know this directly in a readable format as @enistri_devo you have to store it at identity level, its again based on you need of the attribute for other admin or users who can see this.