Complement to fill-paragraph. For LaTeX documents with version tracking, it is best practice to have sentences begin after newlines.

fill-sentences.el

(defun fill-sentences ()
  "Fills the current paragraph or region, starting each sentence on a new line."
  (interactive)
  (save-excursion
    ;; Determine region to operate on
    (let ((beginning-of-region (if (use-region-p) (region-beginning)
                                 (save-excursion (backward-paragraph) (point))))
          (end-of-region (if (use-region-p) (region-end)
                           (save-excursion (forward-paragraph) (point)))))
      (goto-char beginning-of-region)
      ;; Loop over each sentence in the region
      (while (< (point) end-of-region)
        ;; Determine the sentence bounds
        (let ((start-of-sentence (point)))
          (forward-sentence)
          ;; Fill the sentence, breaking at `fill-column'
          (if (derived-mode-p 'LaTex-mode)
              (LaTeX-fill-region start-of-sentence (point))
            (fill-region start-of-sentence (point)))
          ;; Delete extra space
          (delete-horizontal-space)
          ;; If this does not end with a newline, add one and indent
          (if (and (not (equal (point) end-of-region))
                   (not (char-equal (char-after) ?\n)))
              (newline-and-indent)))))))

(add-hook 'LaTeX-mode-hook
            (define-key LaTeX-mode-map (kbd "M-j") #'fill-sentences))