Hello,
currently, the {\an8} tag is not implemented.
It is a very useful tag for situations where the video has hard-coded subs in the original version, but you need to show soft srt-subs in the translated version (Note: this is not an ‘official’ SubRip tag per se, but is often used for this use case nonetheless. VLC on desktop e.g. implements it.)
An example for such a situation is attached. (Note: screenshot is from iPad version; on iPad, the feature is not as much needed due to the aspect ratio allowing for soft-subs inside the black bar on bottom, which is fine. On Apple TV however, one would see a nasty overlayed subtitle with soft on top of hard-coded, which makes reading longer subs very inconvenient.)