The SRT format should never have outputted CODEC_ID_SRT packets in the
first place: SRT is a subtitle format containing SubRip text markup
events. The timing information is part of the format, not the codec, and
thus CODEC_ID_SRT should not exist.

Creating packets with the timing information within the payload only
leads to problem (such as remuxing with timing alteration not working),
especially when the SubRip markup is being used in container like
Matroska in addition to this standalone SRT format.

The main reason the timing line was included in those CODEC_ID_SRT
packets is likely because it contained extra information (the event
position) the codec actually needs. This issue is solved by using the
AV_PKT_DATA_SUBTITLE_POSITION side data type.