There are community-run archives (often found on the official KSP forums or subreddit) that back up old versions of the game for preservation. If you own a license for the game, downloading a backup of the file from a trusted community source (like the KSP Forum "Archived Versions" thread) is a common practice to preserve history.
Kerbal Space Program is a game that offers endless fun and educational value for those interested in space and physics. While it's understandable to look for free downloads, consider supporting the developers by purchasing the game through official channels. Not only does this ensure you're getting a safe and legitimate copy, but it also supports the development of more great games in the future.
Because KSP is still a commercial product being sold (alongside its sequel, KSP 2), version 0.15 is . Searching for "KSP 0.15 free download" on third-party sites or torrents carries significant risks:
Creating an account allows you to register your server(s). Once a server is associated with your account and validated you can start sharing your bans with the community as well as purchasing a 24/7 Rustadmin Online instance. The Rustadmin bans sharing system is very powerful to fight against cheaters and toxic players. When a player is banned from a server and the admin decides to share his ban then anybody else having this player on his server is able to see how many times he got banned from other servers and why.
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I've created Rustadmin in 2014 for my own usage and started to release it in 2015. I had no idea at this point that it would become such a widely used RCON tool and I have been amazed by people I met through this program. Thank you everyone for your support and your kindness, working with server admins is a real pleasure and I'm happy to help people managing their servers.
I don't play Rust anymore (since 2017) but I do still maintain Rustadmin and try to implement the majority of all your requested features.