Dear Rochelle P,
Good day!
Thank you for posting to Microsoft Community. We are glad to assist!
Based on your description regarding "'Show declined events on your calendar' setting missing in Outlook Calendar settings". After thorough research and consults, been advised that the feature you are inquiring about is not yet fully available. Kindly note that this feature is still at the development stage with a scheduled roll out start of November 2023. You may refer or subscribe to this feature in the Roadmap via Microsoft 365 Roadmap | Microsoft 365. Since products do roll out in stages, we do ask for your patience until this feature starts to officially roll out.
Thanks for your understanding! Your patience and cooperation will be highly appreciated. Have a nice day!
Best regards,
Inema | Microsoft Community Moderator