Keep all meeting invites even if declined.
Set the option to keep all meeting invites in inbox regardless of the action taken; even if declined.
Rajib Dutta commented
Give customization option to show declined to invite in Calendar: Yes/ No
Please have two different decline option, with deleting and with keeping the invitation in the calendar.
This would have to be a customizable option, as some users want the opposite; some users want the ability to auto-delete on decline, or decline automatically on delete.