tk0miyaさんのこれ（ https://gist.github.com/tk0miya/603f81c3e168831cca99537949357c7b ）をSphinx 3.2.1でもうごくようにした

ext.py

# TeX出力時の文書構造を変更する

from docutils import nodes
from sphinx.writers.latex import LaTeXTranslator

class AppendixLaTeXTranslator(LaTeXTranslator):
    def __init__(self, document: nodes.document, builder: "LaTeXBuilder",
                 theme: "Theme" = None) -> None:
        super().__init__(document, builder)
        self.is_mainmatter = True
    
    def visit_title(self, node):
        if self.is_mainmatter is False:
            for i, value in enumerate(self.sectionnames):
                self.sectionnames[i] += "*"
        super().visit_title(node)
        if self.is_mainmatter is False:
            for i, value in enumerate(self.sectionnames):
                self.sectionnames[i] = value[:-1]
            self.pushbody([])

    def depart_title(self, node):
        if self.is_mainmatter is False:
            body = self.popbody()
            self.body.extend(body)
        super().depart_title(node)
        if self.is_mainmatter is False:
            sectionname = self.sectionnames[self.sectionlevel]
            self.body.append(r'\addcontentsline{toc}{%s}{' % sectionname)
            self.body.extend(body)
            self.body.append(r'}')

    def visit_compound(self, node):
        if 'preface' in node['classes']:
            self.body.append(r'\frontmatter')
            self.is_mainmatter = False
        if 'appendix' in node['classes']:
            self.body.append(r'\appendix')
        if 'postface' in node['classes']:
            self.body.append(r'\backmatter')
            self.is_mainmatter = False

    def depart_compound(self, node):
        if 'preface' in node['classes']:
            self.is_mainmatter = True
            self.body.append(r'\mainmatter')
        if 'postface' in node['classes']:
            self.is_mainmatter = True

def setup(app):
    app.set_translator('latex', AppendixLaTeXTranslator)