Hi,
The "TO" spot and how it is populated whilst you look for a Name or other info seems to be something for the "Dark Arts".
If I stick in R on my Sierra computer I get this.

I have not spoken to Ryan since about iChat 3
However he is in my Contacts list with one email address. It is iMessages that is presuming that it is a Valid Apple ID that I could send to.
I have a Macbook Pro and a G4 as you can see and these have Contacts cards in my Contacts app.
Those cards have Jabber IDs that I used to use for testing between the 3 computers.
However the Current MacBook Pro I can using is not the one with that Card.
As you can also see in Sierra a Bonjour Connection is still listed.
The RFU is the Rugby Football Union which is the English association for the game of Rugby in this country.
They have an 0800 number and Messages think I could Text Chat (SMS) to this number.
They also have an email for their ticket sales which strangely shows and a possible for Text Messaging.
The RAC (Royal Automotive Club) is one of the largest Roadside Assistant companies over here.
Again this is an 0800 number.
If I do this in High Sierra then I get the same list minus the Bonjour option.
However if I enter +44 to look for phone Number I get this:-

I get people I have actually contacted (Son, daughter and her husband)
The others are a mixture.
Anything that is +44 7xxx xxxx is a mobile (Cell Number)
Anything that is +44 1xxx xxxx is a Land line somewhere.
The list is also longer than the pic as seen by the Scroll Bar
The No Name means they have no Card in Contacts.
Where the app pulls these from other than an historical database of some kind is unknown.
From the Path I listed before:-

I have left this picture quite big so the Path shows full details.
The one file I have listed show in Coverview.
It list a Jabber/Google account and then it's (my) Buddies.
Then it shows an AIM Account then lists the Buddies I have contacted.
The AIM Accounts lists everyone I have ever kept in my Buddy List before AIM was stopped irrespective if they were in my Contacts app.
Now that their is no Active Account for AIM those names are not brought forward to the "To" spot when looking now.
They don't even seem to get suggested and Apple ID despite many being @mac.com or @me.com ending names.
You can edit these soagent .plist with Xcode (free) which might help but you may still have items you can't get rid of in the "To" Drop Down.

10:08 pm Monday; March 5, 2018
iMac 2.5Ghz i5 2011 (Sierra)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
iPhone 6 iOS 11.x and an iPad (2)