I'd seen an 8 speed Shimano freewheel advertised on ebay-it looked like a freewheel rather than a cassette-so they may have had a short market life-or maybe the seller had the brand wrong.
AFAIK, Shimano never made 8-speed freewheels. In fact, they only make a couple of 7-speed freewheel sizes.
Sun Race 8-speed freewheels are still available but 8-speed freewheel hubs are a poor bet due to the excessive length of unsupported axle.