We published paper A before. Now, we are writing the camera-ready version of paper B, which is closely related to paper A. Therefore, we currently reused around 50% of the related work section from paper A. Would this be acceptable? Thanks!
Short answer: No.
Medium answer: It can feel silly to find new phrasings for sentences or paragraphs that say essentially the same thing, but the alternative is unfortunately self-plagiarism. You can and should make a comment like "the following subsection is largely adapted from [other paper]", but even with that, the text should be different enough to not be considered a quotation. There is some wiggle room with technical definitions that can't be rephrased without a loss of clarity, but other than that, err on the side of rephrasing.