shell-things/etc/ssh/sshd_config.d

sshd_config should include something like

Include /etc/ssh/sshd_config.d/*.conf

NOTE: This became supported only at OpenSSHd 8.2 on 2020-02-14. https://www.openssh.com/txt/release-8.2

NOTE also that the first time an option is specified wins.

See also

https://infosec.mozilla.org/guidelines/openssh