Cover Corp or Hololive has set up rules, terms, and guidelines in regards to fan works. Subtitled clips count as part of this.
One of these guidelines is the Cover Corp/Hololive Fan Work Guidelines or Derivative Work License Agreement. These are general rules that you need to follow when making fan work.
Fan Work Guidelines - hololive production
For most clippers and subbers however, you may need to look into the specific guidelines set by Hololive staff. This reply was sent to a subber who tried to upload a full stream sub of Kanata's 3D debut.
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TL;DR: You are only allowed to clip at most ~25% of a video. Ideally, if want to create a video or sub of a stream, you should create either:
Now you may be asking yourself, if I'm subbing a VTuber outside of Hololive, can I skip these guidelines? The answer is most definitely no. Why? Because while agencies like Hololive and Nijisanji have protections and may have direct contacts with YouTube, indie VTubers don't. They are the least protected and the most vulnerable if a subber decides to harm them through mistranslations or full stream uploads. They don't have the same protections nor the ability to take down stuff like that.
As a fansubber, you have to act in good faith when it comes to subbing. Like a real VTuber fan would. Cover's guidelines is a good place to start.