what happens to the metadata of scanned pictures?

For clarification - Photos will embed your metadata in exported files if you tick the relevant box in the export pane. This screenshot is from Big Sur, but I think is same or similar in earlier versions.

However, there is a bug specifically in Big Sur where it fails to do that for .jpg files. So with High Sierra, you should be fine.

You can double check what is included by exporting a file, opening in inspector, and tools>show inspector

Currently the Big Sur Photos app can only embed the IPTC metadata in .png formatted images upon export. So if you want to send image files to your friend with Titles, Descriptions and Keywords embedded export as .png files.

I don't know want you mean by "consequences for the metadata".

From what I remember there's no problem with exporting photos with those two checkboxes selected and getting all of the metadata embedded in the resulting image file. The recipient of the files will need to have an application that can read the EXIF and IPTC fields to get the metadata info.

Use a metadata editor app that embeds Date Created, Description, Location, Keywords etc. into the relevant EXIF/IPTC metadata fields, and supports the image format in which you'll scan. See SnipTag for Mac, it works with JPG, PNG, TIFF and BMP formats. If you plan to use a scanner or a digital camera that doesn't auto-crop the scans, this app also has a batch cropping function.