It depends on what different people mean by masking.
I think to a certain extent, everyone 'masks,' in the sense that they adapt their behaviour to the situation they are in. I wouldn't say it's only autistic people who mask.
In a work situation, people annoy you and you don't blow up, you try to get by, being polite. This could be called masking, surely? You're not showing your true feelings.
If you're at a wedding, normally you'd be quite and listen to the ceremony. You might want to talk to whoever is with you, but you don't. Isn't that a type of masking?
I read about children masking at school and then their behaviour deteriorates at home. But aren't they just behaving as they are expected to at school, and behaving in a less controlled way at home, because they are free to express themselves more or less as they wish?
A previous poster said that she gets by, by watching people and copying them. In some way, everyone does that. We take our social cues from the behaviour of those around us.
At least, I do. Maybe I'm autistic and just didn't know, but I don't think I am.