"We don't want you to play with us anymore. You embarrass us."
Here's the kicker: the note is signed Vicki, Samantha, and Sofie.
As the Reddit thread points out, Sophie's name is spelled with a "ph" on the official NBC website, and spelled with an "f" here — but that doesn't mean this isn't the same character. It's quite possible that Vicki or Samantha wrote that note (Vicki's name is first) and misspelled Sophie's name. If you're an optimist, it's also possible that Sophie didn't even know about the note, and that she didn't know her name was being associated with something so mean.
This may seem like no big deal — especially considering the events in "The Pool" happened 26 years ago — but it's worth wondering why This Is Us would drop hints about Sophie being less than kind to Kate while also establishing Sophie as the love of Kevin's life. We know that Kevin would do anything for his sister, so could this hint that he'll ultimately have to choose between Sophie and Kate in someway? We still don't know what the relationship between Kate and present-day Sophie is, though it's not like they stayed in touch after Kevin's marriage fell apart.
This Is Us is such an emotional rollercoaster that anything seems possible — even Sophie secretly being the worst.