You are missing the following 32-bit libraries, and Steam may not run: libGL.so.1
To solve it just install the following package:
# apt install libgl1-nvidia-glx:i386
Steam should now start without complaining.
Edit #1: As of today, 02/10/2020, on Debian Sid, you have to run the following command instead:
# apt install libglx-nvidia0:i386
Edit #2: As of today, 22/10/2020, on Debian Sid, you also have to run the following command:
# apt install libgl1-nvidia-glvnd-glx:i386