As with most things, there's more than one 'right' way to do these things and a lot of it depends on the child. I was a cynical kid from the get-go and found the idea of Father Christmas illogical. I asked my mother outright when I was four, according to her I said that I wanted her to tell me the truth quite firmly, so she did tell me the truth. Although I don't remember that specific conversation, I do remember having a sense that it would be nice to 'play along' with the tradition and continued going through the motions until I was a teenager, and thoroughly enjoying it 😆
My DS, on the other hand, who's five, WANTS to believe. There is no Santa costume bad enough to shake his determination that it's all real. The Santa that came through our village at the lights switch-on had a beard made out of what looked like cotton wool, and he asked me if he was the real Santa. And then went on to create a story about how one of the kids at his school has seen the actual real Santa and actually, he has a beard just like that, so he must be the real one. Im quite non-commital because I don't want to outright lie, but I can't see how it's really deception to play along with the belief of a child who flits between fantasy and reality due to their developmental stage.
The problem starts, in my opinion, when parents actively do things to sustain their child's belief at the age when they're naturally starting to question things, like making photoshopped pictures of Santa in their living room etc. It's understandable to want your child to stay in that lovely stage for as long as possible. But it's also natural for children to start to emerge from that stage where fantasy and reality merge, and I can see that trying to prolong their belief does cross the line into deception instead of playing along when they start seriously questioning.
I can't really understand the point of view of saying that it's harmful to ask children not to tell their believing friends that Santa isn't real. There are harmful lies and harmless omissions- when I found out Santa wasn't real, I was more than able to understand that there are things we don't say in certain company because it's hurtful, upsetting or spoils a surprise (Santa isn't real, mummy's buying you a jumper for your birthday, that lady has a funny nose) and the difference between that and a proper secret. Although my DS has already been told by other kids, he just scoffs and carries on believing 😆