Thank you for contacting Siteifnity support.
First search these user controls in your solution and check if they are used in your custom widgets or master pages. If these ascx-s are Sitefiniity widgets can search for them in the database is [sf_object_data] table. There you can select all the rows where you control is used like this:
'<put the full type name of your control. Ex: Telerik.Sitefinity.Web.UI.PublicControls.LoginWidget>'
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items