In classic we use %icon% to display the Icon of a file displayed on a page using the File List page type, this tag does not work in Matrix (also tried %asset_icon%) what has it been replaced by?
It hasn't been replaced by anything. You would use custom Type Format bodycopies instead and insert a specific icon that way.
Edited, because I realised you may have been talking about thumbnails, which can be accessed via %asset_thumbnail% keywords.