Description
As of Semver v2.1.0, it will ship with DLLs targeting .NET 4.5.2, .NET Standard 1.1, and .NET Standard 2.0. However, this includes support for versions with are no longer supported by Microsoft. At some point, it is likely to become a problem to support those versions. This also limits which version of the C# language we can use. There may be features in newer versions of C# that would be beneficial to use. When the next major release occurs, we should drop support for some frameworks. We will have to decide at that point which ones it makes sense to drop support for.
MS Supported Version
From what I can tell, .NET Standard doesn't have a separate support lifecycle from the individual frameworks they support. However, MS is already saying that it is "recommend you target .NET Standard 2.0, unless you need to support an earlier version."
For reference, the old .NET standard support chart which is easier to read is reproduced here. Also, here is an article on Demystifying Microsoft .NET Long-term Support (LTS).