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Display preferences settings "Do not notify assignees for" and "Include ICal Invite to Activity Assignees" are only implemented in the form layer
I'm unlikely to work on this but logging that these two display preferences are tied to the form layer and only referenced (either directly or indirectly) via CRM/Activity/Form/Activity.php, and so are ignored when using the api.
This is not recent. I'm sure it has always been this way.