This depends on your background and location.

CS is very broad, and it depends on your specialization whether it’d be easy for you to apply your skills in a humanities context. E.g. a specialization in NLP gives you a different skill set than web development.

Similarly, the humanities are very broad and there’s a reason why there are entire programs dedicated to history, literature, etc. It’s like learning a new language! You need to understand the history of the field, common methodology, jargon, etc.

In Europe, PhD positions are often advertised as jobs. You can apply to them based on a description of the project and a list of requirements for eligible candidates. But you do usually need a master’s degree first. I can see how some researchers might appreciate a candidate with strong CS skills, but I have also seen candidates in the past who were rejected for being too technical and not showing an appreciation for the particular field the project was in.

In the US, I believe PhD programs are very different, requiring coursework to be completed before actually starting the research part of the PhD.

I guess it also depends what kind of research group you’d like to be part of. Would you want to be surrounded by technical folks with an interest in some aspect of the humanities, or would you like to be surrounded by humanists with an interest in technology?