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)