It seems sshd.c is referencing old packet handling functions when DEBUG_KEXDH is defined, specifically: packet_start(), packet_put_cstring(), packet_send(), and packet_write_wait(). It appears it's been this way since OpenSSH 6.8 or so.
Created attachment 3391 [details] fix compilation with DEBUG_KEXDH
This has been fixed and will be in openssh-8.4
close bugs that were resolved in OpenSSH 8.5 release cycle