There is a Discord server for Siv3D users (posts in English are also welcome). Specific channels (chat rooms) are available for discussing Siv3D, asking questions, and showcasing your work. There is also a channel for those interested in the development of Siv3D itself, where updates on development progress are shared promptly and collaborative development of new features is undertaken.
The author of Siv3D checks tweets with the hashtags
#OpenSiv3D. Due to Twitter's system, your tweets may not appear in search results, so if you need a guaranteed response, please reply to @Reputeless. When doing so, to expedite resolution, attach screenshots of the issue or error message, compiler output, etc., and share the section of source code where the issue has occurred using GitHub Gist.
Please post bug reports, new feature requests, and documentation requests to the OpenSiv3D GitHub repository's Issues. You can use any format you want, but if your report lacks information, it may be rejected.