On the 1.10 branch, acceptableRemoteVersions was set to "*" in the @Mod annotation, allowing vanilla clients to connect to servers with the mod installed. Is there any reason the same was not done for the 1.12 version? I compiled a version of Surge that does have it set and installed it on a Forge server, and I could connect with a vanilla client without any problems.

You are correct, not marking it with this property was an oversight on our end. I will fix it up for the next release.

This is fixed in the next release, just waiting on Curse to approve it.