Not official, but I heard Valve is saying that coaches shouldnt have signatures in the game even if they have to stand in for their respective team
1) Why does Valve even collect a 6th man signature at first place then?
2) What if coach becomes a player then?
— Luis Peacemaker (@peacemakercsgo) December 20, 2017
Valve might be considering disallowing coaches ( who are stand-ins from having their own signatures in stickers). This would obviously rob them of an opportunity of making a huge amount of money from sticker sales.