This folder used to be called skills and has been renamed to plugins, however the project file still looks for the old skills folder. This should be changed to "plugins" such that they get copied like they used to.
This is probably true for all of the sample projects.
https://github.com/MicrosoftDocs/semantic-kernel-docs/blob/9f93fdd5fcf114d2ed270beda504689f339b1975/samples/dotnet/03-Chaining-Functions/03-Chaining-Functions.csproj#L24-L26
This folder used to be called skills and has been renamed to plugins, however the project file still looks for the old skills folder. This should be changed to "plugins" such that they get copied like they used to.
This is probably true for all of the sample projects.