Some programs put references in taskbar. Their references will stay in taskbar history even if you uninstall them.

When you open Taskbar  Properties to customize, there are two sections: Current Items and Past  Items.

How do you clear the Past Items?

Open regedit from start /run (Win + R)

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\TrayNotify

Delete the IconStreams and PastIconsStream values from the  registry key.

Then open Task Manager, click the Processes tab, click Explorer.exe, and then click End Process. In Task Manager, click File, click New Task, type explorer, and then click OK.

Taskbar history cleared.