The Signal messenger and protocol.

I dunno what their reason is, but once you get past a given number of pinned chats, it no longer works to keep the really important ones at the top. You'll still have to scroll through to find whatever.

I tend to think 10, on a phone screen, is the very upper useful limit, and I'm not certain anything over 6 really would be

Failure of the 0-1-∞ principle.