Assembly deployment to “bin” virtual folder deprecated in SharePoint 2013
For some time now deploying assemblies in the “bin” folder in virtual folders under inetpub has been frowned on in SharePoint. Well, it’s now deprecated in SharePoint 2013 and an attempt to do this will cause Install-SPSolution to fail.
If you really, really, have to you can use the -FullTrustBinDeployment parameter to allow this behavior. However, you should really look to deploy assemblies to the GAC.