How to install libc6-i386 in Ubuntu 20.04 focal?

N0rbert's comment brought me on the right track, but it still wanted to remove quite a list of packages:

$ sudo aptitude install libc6=2.31-0ubuntu9.2
The following packages will be DOWNGRADED:
  libc6 
0 packages upgraded, 0 newly installed, 1 downgraded, 0 to remove and 14 not upgraded.
Need to get 2 715 kB of archives. After unpacking 0 B will be used.
The following packages have unmet dependencies:
 libc6-dbg : Depends: libc6 (= 2.31-0ubuntu9.3) but 2.31-0ubuntu9.2 is to be installed
 libc6-dev : Depends: libc6 (= 2.31-0ubuntu9.3) but 2.31-0ubuntu9.2 is to be installed
The following actions will resolve these dependencies:

      Remove the following packages:                                                       
1)      build-essential [12.8ubuntu1.1 (focal-updates, now)]                               
2)      g++ [4:9.3.0-1ubuntu2 (focal, now)]                                                
3)      g++-9 [9.3.0-17ubuntu1~20.04 (focal-security, focal-updates, now)]                 
4)      libboost-atomic1.71-dev [1.71.0-6ubuntu6 (focal, now)]                             
5)      libboost-chrono1.71-dev [1.71.0-6ubuntu6 (focal, now)]                             
6)      libboost-date-time1.71-dev [1.71.0-6ubuntu6 (focal, now)]                          
7)      libboost-filesystem-dev [1.71.0.0ubuntu2 (focal, now)]                             
8)      libboost-filesystem1.71-dev [1.71.0-6ubuntu6 (focal, now)]                         
9)      libboost-regex-dev [1.71.0.0ubuntu2 (focal, now)]                                  
10)     libboost-regex1.71-dev [1.71.0-6ubuntu6 (focal, now)]                              
11)     libboost-serialization1.71-dev [1.71.0-6ubuntu6 (focal, now)]                      
12)     libboost-system1.71-dev [1.71.0-6ubuntu6 (focal, now)]                             
13)     libboost-thread-dev [1.71.0.0ubuntu2 (focal, now)]                                 
14)     libboost-thread1.71-dev [1.71.0-6ubuntu6 (focal, now)]                             
15)     libboost1.71-dev [1.71.0-6ubuntu6 (focal, now)]                                    
16)     libc6-dbg [2.31-0ubuntu9.3 (now)]                                                  
17)     libc6-dev [2.31-0ubuntu9.3 (now)]                                                  
18)     libexpat1-dev [2.2.9-1build1 (focal, now)]                                         
19)     libfreetype-dev [2.10.1-2ubuntu0.1 (focal-security, focal-updates, now)]           
20)     libfreetype6-dev [2.10.1-2ubuntu0.1 (focal-security, focal-updates, now)]          
21)     libicu-dev [66.1-2ubuntu2.1 (focal-security, focal-updates, now)]                  
22)     libjpeg-dev [8c-2ubuntu8 (focal, now)]                                             
23)     libjpeg-turbo8-dev [2.0.3-0ubuntu1.20.04.1 (focal-security, focal-updates, now)]   
24)     libjpeg8-dev [8c-2ubuntu8 (focal, now)]                                            
25)     libncurses-dev [6.2-0ubuntu2 (focal, now)]                                         
26)     libpng-dev [1.6.37-2 (focal, now)]                                                 
27)     libpython3-dev [3.8.2-0ubuntu2 (focal, now)]                                       
28)     libpython3.8-dev [3.8.10-0ubuntu1~20.04.1 (focal-security, focal-updates, now)]    
29)     libstdc++-9-dev [9.3.0-17ubuntu1~20.04 (focal-security, focal-updates, now)]       
30)     libtiff-dev [4.1.0+git191117-2ubuntu0.20.04.2 (focal-security, focal-updates, now)]
31)     python3-dev [3.8.2-0ubuntu2 (focal, now)]                                          
32)     python3.8-dev [3.8.10-0ubuntu1~20.04.1 (focal-security, focal-updates, now)]       
33)     zlib1g-dev [1:1.2.11.dfsg-2ubuntu1.2 (focal-updates, now)]                         

      Leave the following dependencies unresolved:                                         
34)     dpkg-dev recommends build-essential                                                
35)     gcc recommends libc6-dev | libc-dev                                                
36)     cython3 recommends python3-dev                                                     
37)     gcc-9 recommends libc6-dev (>= 2.13-0ubuntu6)                                      
38)     gdb recommends libc-dbg                                                            
39)     libgcc-9-dev recommends libc6-dev (>= 2.13-0ubuntu6)                               
40)     libpython3.8-dev recommends libc6-dev | libc-dev                                   
41)     python3.8-dev recommends libc6-dev | libc-dev                                      
42)     python3-pip recommends build-essential                                             
43)     python3-pip recommends python3-dev (>= 3.2)                                        

Thus I forced 2.31-0ubuntu9.2 for libc6-dev and libc6-dbg as well:

$ sudo aptitude install libc6=2.31-0ubuntu9.2 libc6-dev=2.31-0ubuntu9.2 libc6-dbg=2.31-0ubuntu9.2

This command ran smoothly. Steam works now.

One questions remains though: How did I mess this up? How did I end up with libc6=2.31-0ubuntu9.3 in the first place? Should I make a new question for this?