How to configure "DiskImageMounter" like in /etc/fstab?
You are correct. There is no /etc/fstab file, but the use of the /etc/fstab file by macOS has not be removed. You just have to create this file yourself.

An example of a possible lines in the /etc/fstab file are given below.

UUID=F21AD81B-B114-456C-B2A0-BF4452E4842D none apfs rw,noauto
LABEL=Macintosh\040HD none apfs rw,noauto


A description for the fields can be found by entering the command man fstab.

You may want to refer to this question: How to prevent auto mounting of a volume in macOS High Sierra?

When using ls -l /etc/fstab to determine ownership and permissions, you should get the same as shown below.

-rw-r--r--  1 root  wheel  123 Dec 31 08:48 /etc/fstab


UPDATE (provided by iggie)

A modification of this answer can be used for ssh keys on an external FAT or ExFAT drive (e.g. mounted via VeraCrypt or whatever). Add the following line to your /etc/fstab file, even if it doesn't exist: 

LABEL=PRIVATE none msdos -u=501,-m=700


This assumes your user ID is 501 (try ls -n ~) and the label on your FAT or ExFAT volume is PRIVATE.