ffmpeg -encoders | grep -i "^ [S]"
returns:
... S..... ssa ASS (Advanced SubStation Alpha) subtitle (codec ass) S..... ass ASS (Advanced SubStation Alpha) subtitle S..... dvbsub DVB subtitles (codec dvb_subtitle) S..... dvdsub DVD subtitles (codec dvd_subtitle) S..... mov_text 3GPP Timed Text subtitle S..... srt SubRip subtitle (codec subrip) S..... subrip SubRip subtitle S..... text Raw text subtitle S..... ttml TTML subtitle S..... webvtt WebVTT subtitle S..... xsub DivX subtitles (XSUB)