N.B. This procedure uses the same Windows / Outlook account used for storing the To Do information.
See this link to Main ‘IntoWindows’ Help Page which is a good read. It talks about the evolution of "To Do" [from Wunderlist and it's developers] and highlights some omissions and workarounds.
If items are accidentally deleted within “To Do” there is currently no method to easily restore them, but this may come later. However as you already have a Microsoft Outlook account [necessary to run the “To Do” service] you also have access [using your same credentials] to their email tool “Outlook” [even if you don’t use it normally] from where you can carry out the restoration of deleted items.
Personal Notes & Lists:
If you delete Notes from a List you own you can recover them using the Web app. However if you delete a List it is a bit more complicated and you need to use a facility within Outlook [under Windows] to recover it.
Shared Notes & Lists:
You can deliberately remove a shared Note from within a shared List. This will remove it from your view and, unfortunately, the views of all other 'shared' members. The Note itself is not actually deleted and will still be available to the owner.
You can't delete a shared List [owned by someone else] you can only opt to leave it thereby removing it from what is visible under your To Do set up. When you leave a shared List it not deleted either and is still available to the owner.
Both shared Notes and Lists that have been removed in this way can be found in the owning Microsoft account under "Deleted Items" of Outlook / Tasks from where the owner can re-instate them and offer to share them again.
This is explained more fully in the Main ‘IntoWindows’ Help Page
SOME HANDY LINKS - to save you looking :-))
“Deleted Items” within your / the owners Outlook
"Tasks" folder within your / the owners Outlook