McLaren have stated that they are sticking with their drivers Carlos Sainz and Lando Norris for 2020, being the first team to do so this year.
Both Ferrari drivers are under contract for 2020. There have been rumours circulating that Vettel may retire at end of 2019. Vettel denies the rumours. Ferrari are rumoured to have been checking out Bottas' and Ricciardo's availability in case plan B is required if Vettel pulls the pin.
Verstappen has a contract with Red Bull until end of 2020. His father is on good terms with Toto at Mercedes and regular Verstappen/Mercedes rumour surface in the press, but counter to this is the good working relationship that Hamilton/Bottas seem to enjoy. I expect that Max/Lewis pairing wound a return to the old Niko/Lewis days for Mercedes, so who knows.
Red Bull are in a bit of a spot with their clearly second driver Pierre Gasly has struggled this season and they are currently short on drivers in their driver pool who are ready for F1. Although Red Bull have a history of being fairly ruthless for under performers, they state they are sticking with Gasly and giving him every assistance to help him succeed.
Hamilton has a contract with Mercedes until 2020. Bottas has a contract until end of 2019 with one year option for 2020.