Amusingly, this came to me by initially misreading the title of this thread:

https://www.reddit.com/r/HPfanfiction/comments/vq7eav/harrys_childhood_becomes_public_knowledge_in_the/

When Harry was younger, in his pre-Hogwarts days, he made a close friend at Privet Drive that no-one knew about.

Since he went off to Hogwarts for most of the year, and a lot of his Christmas times were spent at places like the Burrow or Grimmauld, distance between the two grew.

However, somehow the secret is leaked and their identity becomes public knowledge.

The repercussions that ripple throughout the wizarding world could be anything, positive or negative: fear, disbelief, terror, amazement, et cetera. Or just plain "oh well" if they were a muggle.

Harry's friend could be anyone or anything -- this could work well for crossover settings, I think. I can just imagine everyone's horror if his friend was Doctor Doom's son or daughter, or something, or an alien, fae, or Lovecraftian abomination.