Please, backport https://svn.apache.org/viewvc?view=revision&revision=1870095 and https://svn.apache.org/viewvc?view=revision&revision=1870097 from trunk to httpd-2.4.x. This allows using TLS 1.3 client cert authentication for POST requests.
This would be great to have for Debian & Ubuntu.
This was done in r1881713 and is resolved in 2.4.57+.