Once activated, admins will not see deprecation notices. If you want to see them, go to Admin > Configure > Settings > Quiet Deprecation, and add your user to the list. Afterwards, you'll be able to see the notices.

The notices are filtered after rendering the page/default and page/admin views, and by looking for the expression /

Deprecated in \d[^<]+

/, so this may not cover all cases (notably AJAX views).

Also, hiding the notices doesn't mean they don't exist. You should aim to get rid of them because calling deprecated functions is slower and fills your error log with junk.