A lot of the stuff I hear people describing as masking is stuff that a few decades ago would have been called professionalism, or manners, or charm, or empathy.
When I'm meeting with representatives of a client, and one of them makes a suggestion that I know won't work, and I refrain from instantly and publicly shooting down their idea and instead I make a show of considering it, then ask them a probing question or two that let them realise the problem with the idea themselves - am I masking? Or am I merely being professional?
When I see something worthy of a compliment and I rehearse it in my mind to make sure it isn't objectifying or creepy, and that it's personalised to them and shows I paid attention to and understood the thing I'm complimenting - am I masking? Or just being charming?
When I'm socialising with people who are talking about their struggles to lose weight, and I in many years going to the gym have struggled to gain weight or visible muscle, but I keep my mouth shut rather than trying to build rapport about our shared inability to reach our target weight because I've tried it and I know it doesn't work - am I masking? Or just showing a normal level of social skills and empathy?
I think maybe you're misunderstanding what masking is. Masking isn't just not being an asshole. I'll give you some examples.
My face is usually flat, and my tone is usually monotone. Most people modulate their face and their tone naturally and don't have to think about it. When I socialize, I have to think about my face and my tone constantly. Yes everyone is doing this at some level, but for autistic people it takes as much effort as the thing the socializing is meant to accomplish, so that part cannot be enjoyed or suffers as a result of the intentional masking.
Another example is I have echolalia, which means sometimes I have an involuntary urge to repeat things that I heard earlier. I also have various tics where I move my body in wild ways that tend to make people uncomfortable or look at me in strange ways if I do them in public. When I'm socializing, I suppress (mask) all of these urges. I'm sure that most people suppress urges to move their body and make noises in public, but not in a way that autistic people suppress these urges -- again it comes down to it being a conscious effort that causes discomfort. I would compare it to having to sneeze for an entire meeting; all you are going to think about is having to sneeze, and the disruption you will cause if you sneeze and everyone will look at you. But it's not a simple sneeze, it's an action that everyone will talk about and gossip about later, something that will ostracize you.
Another thing I do when masking is more like mirroring. I'm constantly monitoring the situation to figure out how to respond, since I can't use the content of people's words I have to use contextual cues to figure out the appropriate way to respond. For instance I can know a joke is being told by the way someone is telling it, but I might not be able to tell you why it's funny based on the content. Still, I can laugh at the joke but I'm not laughing because I found it humorous, I'm laughing because I discerned the appropriate social place to laugh. And yes people laugh at jokes they don't understand all the time, but for me this extends to everything. I don't know when people are being mean to me, or flirting with me, or scamming me in real time. It takes days, sometimes years of reflection for me to figure out the actual social context of a situation I've been in.
I'm sure all this is true for many people, as I can't imagine socializing is easy for anyone. I do see people who can effortlessly talk with anyone and everyone, and I wonder how they do it, and if they face the same kinds of struggles as me when they go home and decompress. Do they stay awake all night replaying every social interaction from the night before? Do they have to spend days recharging before they can go socialize again? I don't know. But for me it's true to a degree where it makes me never want to go outside again. And that's what makes it a disorder instead of just a quirk or something everyone does.
When I'm meeting with representatives of a client, and one of them makes a suggestion that I know won't work, and I refrain from instantly and publicly shooting down their idea and instead I make a show of considering it, then ask them a probing question or two that let them realise the problem with the idea themselves - am I masking? Or am I merely being professional?
When I see something worthy of a compliment and I rehearse it in my mind to make sure it isn't objectifying or creepy, and that it's personalised to them and shows I paid attention to and understood the thing I'm complimenting - am I masking? Or just being charming?
When I'm socialising with people who are talking about their struggles to lose weight, and I in many years going to the gym have struggled to gain weight or visible muscle, but I keep my mouth shut rather than trying to build rapport about our shared inability to reach our target weight because I've tried it and I know it doesn't work - am I masking? Or just showing a normal level of social skills and empathy?