»DownSub is a FREE web application that can download subtitles directly from Youtube, VIU, Viki, Vlive and more. We supports downloading all subtitles/captions formats such as: SRT, TXT, VTT.
DownSub doesn't force our user to download or install any type of extensions or third party software. We provide an online method to download subtitles by just entering the URL of the video and clicking Download.«
I'd wish there were a CLI variant, though.