I did read [1], and I understand `nvidia-legacy-390xx-driver` is
deprecated. Anyway -mostly for reference- here is my lame attempt [2]:

% cat /etc/apt/sources.list.d/unstable.list
deb http://deb.debian.org/debian unstable main contrib non-free
non-free-firmware

% cat /etc/apt/preferences.d/unstable
Package: *
Pin: release a=unstable
Pin-Priority: -1

% sudo apt update && sudo apt install nvidia-legacy-390xx-driver -t unstable

Leads to:

% dmesg
[...]
[   11.166966] Initializing XFRM netlink socket
[   11.175387] traps: gdm3[4753] trap int3 ip:7fdb9f9b921a
sp:7ffcbd306b00 error:0 in
libglib-2.0.so.0.8400.4[6721a,7fdb9f971000+a1000]
[   12.582986] traps: gdm3[4916] trap int3 ip:7f89d460221a
sp:7fff7699ddf0 error:0 in
libglib-2.0.so.0.8400.4[6721a,7f89d45ba000+a1000]
[   22.784285] logitech-hidpp-device 0003:046D:2011.0007: HID++ 1.0
device connected.

Full dmesg attached in case that helps bots.