1.Yes, SharePoint App-Only (Azure ACS) will be indeed deprecated in near future as the Azure AD should be used for the app registrations.
SharePoint App-Only (Azure ACS) will stop working for new tenants as of November 1st, 2024 and it will stop working for existing tenants and will be fully retired as of April 2nd, 2026.
2.Client ID and Client Secret for each site collection is unique. So, you have to create for XYZ separately.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.