Prominent Valve developer Pierre-Loup Griffias announced on Twitter that Valve will not officialy support or recommend Ubuntu starting from 19.10:
https://twitter.com/Plagman2/status/1142262103106973698
This means there is an empty vacuum over which distribution willl be supported by Steam. We know 100% that Debian will offer multiarch support in Buster, so maybe reaching out to Valve over official Debian support might be good. In any case, it is up to Valve to make a decision.
As a reminder, Ubuntu is dropping 32-bit, including multiarch support starting from 19.10. I will refrain from ranting about this, because it is know that Canonical is a "partner" of Microsoft, so them doing something harmful to Linux is not a surprise. Even if they retract their decision, the damage is already done and the trust is lost.
So, go and reach out to Valve about this.