“Crocs” (cheap plastic shoes that are more like slippers than shoes) are superinformal. So are canvass shoes or the old-fashioned cloth sneakers (which are enjoying renewed popularity).
One can get very formal looking non-leather shoes that look just like leather even though they are”man made” (meaning, glorified plastic). Then everyone will come to you and remind you that wearing leather shoes is prohbiited. You can’t win.
Nu!