I ever saw it discussed. It’s so old post (it’s hard to find. I tried to search it again, but not find it). Hope you can understand WhiteNoise.
Writing non-existing pages means you type [[link with no page]] which is not existed .md file (just link). People usually use this because they’re still writing in the current page and want to make [[page]] for later (think it as potential page).
If “|” make an autocomplete, it will be hard to make alias for [[link with no page]] because it will create autocomplete with existing page, not with non-existing page.
For workaround, they will need to press “Esc” to avoid autocomplete with existing page and this creates a writing friction. That’s why “|” doesn’t create an autocomplete.
Different with #, ^, which people will always want to link to existing page.
Sorry for my bad English, hope it’s help.