It is not directly julia, its VS Code extension. They are just png files(or some other image format) in a temporary folder. The extension hooks itself in displaying of plots and saves them as image files in a temporary folder. I think there was clear plots somewhere but I do not remember.
I found this reference. It says Shift+Delete should delete all plot history.
Yes, VS Code’s plot pane has a “trash” icon that you can use to delete the current plot; and moreover there is an option to delete them all at once (you have to focus on the plot pane to see the icons and the “dots” menu):
Ah ok … This is very strange!
I can’t find this option in VS Code on my notebook (Unix OS).
But these option is available in VS-Code and Windows.
Thank you very much 
!