Varies by airline and country, I did a big Europe tour last week and here are my findings:
KLM ATL-AMS: masks optional
Vueling AMS-FLR: Not only masks are mandatory but you need the FFP2 masks (how convenient there is a vending machine next to the check in counter selling them at 6Euros a pair
)
Austrian FLR-VIE-BUD: masks required but not super strongly enforced
Swiss BUD-ZRH-SFO: masks optional
Luckily the long flights is optional, which makes sleeping a little better. The short flights I did not mind