Let's assume some of the info is correct, right.
That a changed happened around 1992, when his mother died (there should be grave records for someone named Alter that died in Doncaster in 1992 btw).
This means from the 90s into the early 2000s, according to the Brazilian worker, David Firth and the usenet post, someone would've had to capture him on film.
Look at this video from 1995 at a Doncaster market
Someone like ssb9 would stand out like a sore thumb in an environment like that in the 90s-00s.
If he allegedly were known among the kids in the 90s-00s, we can assume he went in costume at daytime. If we assume the sighting in the brixton video is true, and he's wearing casual wear - he might have sometime between the two points become more cautious. But still, he would stand out more in Doncaster than in Brixton.
Like this video of a Doncaster market uploaded 9 years ago