Comment 5 for bug 1405424

xml2enc is available in the packages:
 $ dpkg -S /usr/lib/apache2/modules/mod_xml2enc.so
   apache2-bin: /usr/lib/apache2/modules/mod_xml2enc.so

And it gets enabled automatically if needed:
$ a2enmod proxy_html
  Considering dependency proxy for proxy_html:
  Enabling module proxy.
  Considering dependency xml2enc for proxy_html:
  Enabling module xml2enc.
  Enabling module proxy_html.
  To activate the new configuration, you need to run:
    systemctl restart apache2

I don't know when this was resolved as I came by this bug parsing old non-updated bugs. But this seems resolved to me at least in Bionic and later.