The Visual Studio side of the equation is tooling. If Visual Studio 15.7 has the ASP.NET Core 2.1 tooling included in the release, then yes you can use it to develop ASP.NET Core 2.1 apps. However, if the ASP.NET Core 2.1 code is still in flux, that means the tooling could change as well, so until the framework release is final, it's still a better idea to continue using the preview build of Visual Studio.
It's worth noting that ASP.NET Core 2.1 is conspicuously missing from the release notes. It's very likely that that support was stripped from 15.7 to allow the release of other necessary features that were ready to ship. I haven't updated myself yet, so I can't say for sure, but your question may be moot.