Hello @Radim Langer
VS2019 does not have an option to exclude the folder name e.g. if we have a namespace of Blog.Classes creating in the Class folder Operations.cs the namespace will be Blog.Classes.
If you want Operations.cs in Blog namespace a work-around is to create Operations.cs in the root folder of the project then in solution explorer drag-n-drop the class file into a Blog.Classes.
So your current idea and the work-around is your choice to make.
If you have Resharper installed, right click on a folder e.g. Classes, select properties then for Namespace Provider change the value from True to False then the root namespace is used rather than appending the folder name to the namespace.