The guiding principle for the 2021/22 ski season in the Alps and the Pyrenees is that as long as face masks are mandatory on trains and other forms of public transport, they are also mandatory on ski lifts too. Snoods and bandannas can usually be counted as masks, provided they cover the mouth and nose.
But rules and regulations can change fast, and different countries may adopt different approaches, so check with your resort’s tourist board before setting off for the slopes.