The proper way to dispose of a flag is to burn it when it is no longer serviceable and then bury the ashes. It is not to be tossed into the trash.
The flags in question were not covered in what I saw regarding disposal, but if they weren’t to be used then I’m guessing the same procedure would be applicable.
To bring them out on stage and show people what the Democrats had planned for old glory would be a powerful statement indeed.
You can drop old flags by your local VFW and they will care for them.