Hi,
I have seen such files for training purposes but cannot find in JOSM UI how to set this in a new .osm file? Does someone know the tip? Is there a resource about it somewhere?
In JOSM, you can enable the upload warning for a file by right-clicking on its entry in the layer selector (it’s on the top right of the JOSM window by default) and choosing the “discourage upload” option. Trying to upload that file will then display a warning. This option may require expert mode to be enabled in JOSM, not sure.
It’s also possible to go a step further by using a “never upload” flag, but that requires manually editing the .osm file in a plaintext editor as explained here: https://josm.openstreetmap.de/wiki/Help/Action/EncourageDiscourageUpload