A limitation disappears
Up through iOS 26, searching in the Messages app worked well only if you knew the contact's name exactly as saved in the address book. Searching a conversation by bare phone number — useful when you receive a message from an unsaved number — was impossible. With iOS 27, search also accepts the number directly and any nickname assigned to the contact.
Why it matters
This is one of those features that seems trivial until you need it: someone texts from an unknown number, then you save them, but the old conversation disappears from search. Or you have a contact saved under an informal nickname and name-based search finds nothing. iOS 27 closes both gaps. The feature is documented in the list of 263 keynote changes covered by iClarified, and confirmed in MacRumors' live blog.