How would you clean/prepare a flexible/magnetic mat (e.g. Ender 5) as compared to a glass bed?
I have the WhamBam system which uses a PEX layer over flex steel (which sticks to a magnetic sheet on the printer bed). To clean old material off, I use a "brass sponge" intended for cleaning soldering iron tips to remove the old plastic, then give it a wipe with a paper towel with some isopropyl alchohol (I have 99.99 anhydrous on hand as I use that for cleaning printed circuit boards as well).

The brass sponge is fairly soft, does a good job of grabbing the old plastic without tearing up the PEX layer.