Forum Discussion
Confidentiality or Terms of Use Notice Dialog/Popup on SharePoint
You have the possibility to add an authentication context to a container label in which you can be very specific only for that particular site. You specify the authentication context from AAD in the container label setting. And that AC can have a terms of use for that site. Bear in mind there are some limitations (still) when using AC's for Teams.
Authentication context
https://learn.microsoft.com/en-us/azure/active-directory/conditional-access/concept-conditional-access-cloud-apps#authentication-context
Container labels
https://learn.microsoft.com/en-us/microsoft-365/compliance/sensitivity-labels-teams-groups-sites?view=o365-worldwideIf you don't want to set up the site with other external access and sharing controls you can simply add an AC using Set-SPOSite.
Set-SPOSite -Identity https://contoso.sharepoint.com/sites/research -ConditionalAccessPolicy AuthenticationContext -AuthenticationContextName "MFA" (example).
The Conditional Access TOU is the only method I know of. When you set up a new TOU, there is the option at the bottom to target your CA policy to Users and Apps. You should be able to target to groups as well. You test this - but only if your SPO sites are permissioned by AAD groups as opposed to SPO groups. That's the only way I could see this working for you in such a specific manner.