This Jellyfin plugin allows to exclude files and folders from a library can using a .jfignore File.
Files are excluded relatively from the ignore-File, which can be placed in every folder of the library.
Go the the Jellyfin plugin page (in the admin section).
Add a new repository (in the repository section).
https://raw.githubusercontent.com/virtualmarc/jellyfin-jfignore-plugin/master/manifest.json
Select ".jfignore" and install (in the Catalog section).