Many of the issues in this repo were originally created in the dotnet/toolset and dotnet/cli repos, then migrated here to the dotnet/sdk repo.
In this process their ID numbers inevitably changed. If you have the old ID number, you can find the new ID number by simply navigating to the old URL: your browser will be redirected to the new URL.
For example, https://github.com/dotnet/cli/issues/10 redirects to #4288.
In some circumstances you may have the new URL and want the old one. For example, if you are looking at a migrated issue and wish to search your email or chat logs for a discussion that referred to the old URL. In that case, you can use these mapping files to look up the old ID and find the old URL.