I don't like the available subtitle fonts in Kodi and want to add my own .ttf file.
How do you do that? I use both OSMC and LibreELEC.
On OSMC copy your .ttf font file to /usr/share/kodi/media/Fonts/. On LibreELEC the path is instead /storage/.kodi/media/Fonts. For this you need to activate SSH login, and use a tool like WinSCP to perform the copy.
This should suffice to make your font show up among the available Kodi subtitle fonts in the Kodi user interface.