Running 3.6.9 (7523) on High Sierra with an EyeTV Diversity OTA, with no issues.
When I play a show recorded with EyeTV in EyeTV, I can turn DVB subtitles on and off, so the application clearly has the ability to handle such subtitles.
However, if I export the show to a native format (h.264), only audio and video are exported.
Ideally, I would wish for EyeTV to be able to export subtitles, either in a .mp4 or .mkv container.