I was pretty sure I have seen this in sed(1) already, but I couldn’t find it anywhere, so I’ve been nerd-sniped into doing it myself:

:b
p
s/$/0/
s/^/,0/
:a
s/,\(.\)\(.\)\(.\)/\1\2\3,\2\3/
s/111,/o,/
s/110,/o,/
s/101,/o,/
s/100,/x,/
s/011,/x,/
s/010,/x,/
s/001,/x,/
s/000,/o,/
t a
s/,.*//
y/xo/10/
t b

Example run:

% echo 00000000000000000000000100000000000000000000000 |sed -f rule30.sed |head -20 | tr '01' ' #' 
                       #                       
                      ###                      
                     ##  #                     
                    ## ####                    
                   ##  #   #                   
                  ## #### ###                  
                 ##  #    #  #                 
                ## ####  ######                
               ##  #   ###     #               
              ## #### ##  #   ###              
             ##  #    # #### ##  #             
            ## ####  ## #    # ####            
           ##  #   ###  ##  ## #   #           
          ## #### ##  ### ###  ## ###          
         ##  #    # ###   #  ###  #  #         
        ## ####  ## #  # #####  #######        
       ##  #   ###  #### #    ###      #       
      ## #### ##  ###    ##  ##  #    ###      
     ##  #    # ###  #  ## ### ####  ##  #     
    ## ####  ## #  ######  #   #   ### ####    

Awesome 👏