Thanks for the answer. RoyalMix just informed me that they have fixed the problem. Validation takes place on the server, and then all information is deleted, so everything is safe. Information on the site has been updated (FAQ).
The public IP address posted above is still accessible.
And also I have found that the IP belongs in Hostinger's namespace and ASN.
Hostinger is a very anti-mixer web hosting company (they banned my site from their platform), so this is another issue that needs to be rectified.
Ultimately, switching hosts and fixing the DDoS-Guard IP leakage should mitigate this.
Also it appears that there's an SSH server running on that port that is not properly firewalled - generally speaking it should only be accessible from IP addresses inside the developers' ISP. Please raise this issue with them too.
Mixer security is very very important because they are very attractive targets for hackers.
Edit: HOLY SH!T! It's worse than I initially thought.
Apparently, Censys has IP addresses for
all mixers and many other sites, even the ones behind cloudflare.
You have definitely got to use the Tor mirrors, but it only works if it's on a different server.