I have resolved the issue by completely removing all Microsoft.Bcl
and Microsoft.Bcl.Build references as @nozzleman suggested in comments.
Although no dll references were present in the
References list in solution explorer, there were entries in package.config for those 2 packages as below:
<package id="Microsoft.Bcl" version="1.1.10" targetFramework="net461" />
<package id="Microsoft.Bcl.Build" version="1.0.14" targetFramework="net461" />
I did a search (Ctrl + F) in full solution for the terms Microsoft.Bcl and Microsoft.Bcl.Build and removed all those entries from packages.config and also from .csproj files which had entries like below:
<Import Project="..\packages\Microsoft.Bcl.Build.1.0.14\tools\Microsoft.Bcl.Build.targets" Condition="Exists('..\packages\Microsoft.Bcl.Build.1.0.14\tools\Microsoft.Bcl.Build.targets')" />
<Target Name="EnsureBclBuildImported" BeforeTargets="BeforeBuild" Condition="'$(BclBuildImported)' == ''">
<Error Condition="!Exists('..\packages\Microsoft.Bcl.Build.1.0.14\tools\Microsoft.Bcl.Build.targets')" Text="This project references NuGet package(s) that are missing on this computer. Enable NuGet Package Restore to download them. For more information, see http://go.microsoft.com/fwlink/?LinkID=317567." HelpKeyword="BCLBUILD2001" />
<Error Condition="Exists('..\packages\Microsoft.Bcl.Build.1.0.14\tools\Microsoft.Bcl.Build.targets')" Text="The build restored NuGet packages. Build the project again to include these packages in the build. For more information, see http://go.microsoft.com/fwlink/?LinkID=317568." HelpKeyword="BCLBUILD2002" />
Those lines were appended when it was installed before and as someone manually removed references by pressing Delete key, they were not removed.
After cleaning those 2 items from everywhere the warning is gone from build.