I think it is safe to say now that Koopa's selective mutism is a thing of the past.
About a month and a half ago he started talking freely in most settings, chatting to friends at daycare and on playgrounds, and even occasionally replying to grown ups he doesn't know too well, like his dentist. If you just met him for the first time in July, you probably wouldn't even know he ever had a talking problem.
Not just that: he initiates conversations. One day a few weeks ago I witnessed him introducing one boy to another on the playground, without anybody's prompting! Another time, from the safety of our first floor balcony he yelled "Hi!" to a kid he never saw before, and introduced himself. The kid ran away without replying, but Koopa didn't seem discouraged: he just looked mildly surprised but not upset: "Mama, I said hello to the boy but he didn't reply!"
I'm not sure what helped and why he had decided to stop talking for a few months in the first place. Just like nobody really knows why some kids develop anxieties and others don't. Maybe it was his way of dealing with some family stress that now subsided, or maybe it was just a developmental stage that would have happened anyway. As with most other things in life, we will never know for sure why and how exactly it all unfolded. And I'm not sure what helped it. The only thing we did actively is try to offer our friendly presence and support when he wanted to interact but was anxious ("Do you wanna say it together? In whisper?") and always give him the opportunity to interact non-verbally. But mostly I think things just unfolded on their own in his case. And I'm so glad to see him talk again. I don't think I was as excited when he first started talking as a toddler as I am now!
And now we're in a new city. A new environment, new playgrounds, new kids. And in September he's going to start school. And I'm not even worried about him. I know he will be fine. I know that even if it takes a little time, he will talk and find new friends and will adjust. And maybe I'm secretly hoping that the months of his SM and his overcoming it gave him some immunity against fear of interactions in the future? Maybe now he will always be able to yell "Hi!" to a stranger and not be discouraged to hear nothing in response? Nobody knows, but this is my motherly hope for him.