Same problem: it appears that if you select the YouTube preset, there is no "export subtitles" option, while if you select a custom mode, you have the subtitle section, but you are not sure whether your export will be optimized for YouTube. Perhaps not; in fact I used a custom setting, with subtitles enabled, and "burn into video" option selected, but the quality of the video on YouTube is rather mediocre, both in resolution, and in smoothness: it's rather swinging, as if there was some problem in adapting the frame rate. Original footage: 60 fps, 4K; YouTube video: 24 fps, full HD.
You can see for yourself: