Which version of iOS causes Messages.app to combine message threads with a single contact with multiple different phone numbers into one? Assume the following:


You add a contact (let's refer to them as "Person A") to your iPhone and add two separate numbers for them - one entry labeled "Home", and the other as "Work"
You open Messages.app, and compose a new message to Person A's Home phone number - this creates a new message thread, because up until this point you hadn't ever messaged them previously
Person A then replies to your message, but using the number that you have labeled as "Work"


Whenever the above scenario was encountered, I can recall that in older builds of iOS that two message threads would have been created - one for each number (in this case, Home and Work). Now, however, iOS appears to combine the conversations from both numbers into a single message thread because it recognises that despite being separate physical phone numbers, the contact is still the same person.

So - my question. Would somebody be able to clarify which version of iOS introduced the new behaviour described above?
The merging of message-threads with the same contact was introduced in iOS 12. But Apple didn’t officially announce it, what caused a lot of confusion, because sometimes it also merged different contacts: Forbes: Apple iOS 12 Has An Embarrassing Problem