New in Marvin 1.51.0 on web/desktop (and 1.51.2 on mobile) is a Trash Area.  You can use it to recover deleted Tasks, Projects, and Recurring Tasks.  In the future it will also be possible to recover Saved Items, Smart Lists, Custom Screens, etc.

On the web/desktop, open the ☰→Trash menu at the top-right of your day list.

On mobile, first tap the ☰ at the top-left of your screen to open the sidebar.  Then tap Account / Settings at the top of the sidebar, and finally Trash.

On web/desktop, hover over an item with your mouse.  Two buttons will appear - one to restore, and another to permanently delete.  Be careful what you click as these actions cannot be undone!  On mobile, tap an item to be prompted whether you wish to permanently delete an item or recover it.
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Items will remain in the trash until you delete or recover them.  There's also an "Empty Trash" button for your convenience.  Having many items in your Trash shouldn't affect Marvin's performance (unless you have many thousands of items there), nor does it have any effect on your sync performance.

Items in your trash are not synced among your devices.  That means that you must recover items on the device where they were deleted.  This also means you should be careful when you clear your browser's cache or uninstall the app.

No, Trash items are not currently backed up.