The actual list at https://osgameclones.com it's so huge that literally invalidates any point stated by HN commenters ignoring that, yes, Giana Sisters for the C64 was taken down but after that several Mario shareware clones existed for PC and a few years later we even got Supertux and, in the 2000's, Secret Maryo Chronicles which got renamed in order to avoid any issues with "Mario" as a TM, but the gameplay was 99% the same of Super Mario World.

On software recreating something propietary:

- FreeBSD, NetBSD, OpenBSD... vs AT&T Unix.

- GNU+Linux or GNU+Hurd against propietary Unix kernels.

- Coreutils+Findutils+Sharutils... every tool reimplemented being propietary.

- Bash, GAWK, GCC, binutils, Clisp, SBCL, GNU GCJ+Classpath, Red, FreePascal+Lazarus, GNUStep+WMaker, LessTif, EMWM+Motif vs Irix' Maxx Desktop (still propietary, and from the 90's) Gaim, AMSN, 7zip, Haxima+Nahzgul (and Ultima it's still being sold at GOG), Supertux2, Supertuxkart, ReTux (very Wario like), Hexoshi (Metroid), SMC (Super Mario World), Pingus (Lemmings), XMMS/Xine cloning WinAmp and maybe PowerDVD (Xine with skins), WordTsar for Wordstar, Nano for propietary Pico (and Alpine for propietary Pine), BSD vi and Amiga vim for maybe propietary vi under AT&T/commercial Unix, Lincity and Lincity-NG for Simcity, FreeCiv against Civilization (it can use both the OG Civ rules and their custom ones), Frozen Bubble (I think the level set it's from the Neo Geo release), KGoldrunner and such for Lode Runner, Kapman for Pacman, every BSD shipping Tetris and Boggle, GNU Octave for Matlab....

The list example for both software and games being just reimplementation/clones of propietary tools goes on an on. Even DOS had propietary clones which had to reimplement the same interface as MSDOS because if not the tools written for it would just crash. Same commands, same output, same formating tools, same memory layout, they ran the same DOS binaries and drivers...

An update on Steam / GOG changes for OpenTTD