The Welcome Window items are separately held from the OS recent file listing.
It has not been exposed where SketchUp keeps the Welcome Window file listings.
For Mac OS editions there is a menu command to clear the recent file list for that particular SketchUp version.
For Windows OS there are two recent lists:
* For recent Windows versions with JumpLists, you can remove individual items from the JumpList by going to the SketchUp shortcut (in the Start menu or the taskbar if you also pinned it there.) Right-click the icon to reveal the JumpList, and then right-click a SKP file that you wish to remove. Choose “Remove from this list”.
* SketchUp still internally uses it’s own legacy “Recent File List” registry key. To remove a particular file item, you need to note the position of the file you wish to remove. (The recent files are prefixed with a number.)
So, for example to remove the recent file numbered as 8 from a command shell, with SketchUp closed, use:
reg delete "HKCU\SOFTWARE\SketchUp\SketchUp 2022\Recent File List" /v "File8" /f
WARNING: SketchUp stops enumerating files to add to the menu when it finds no entry for the number it is looking for. So if you were to remove “File1” from the list, SketchUp would not populate the Recent Files submenu at all. If you were to remove “File5” out of 8 files listed, then the Recent Files submenu would only have 4 files listed (skipping everything beyond 5.)