Check if files in list (TXT) do exist (and which files don't exist)

list-file-compare.sh

#!/bin/bash

input="$1" #The file to read from given as first argument
#File content looks like this:
#/a.txt
#/subdir/b.txt
#File needs to have a newline at the end to have the last line processed!
while IFS= read -r line #Repetitively read lines
do

  line=$(echo "$line"|tr -d '\r') #remove \r character
  line=$(echo "$line"|tr -d '\n') #remove \n character
  
  #https://mywiki.wooledge.org/BashFAQ/089 if you run another command here
  if [[ ! -f ".$line" ]]; then #if file at .$line (dot for current directory) does not exist, then
    echo "$line" #output the path of the missing file
  fi #end of "if branch"
  
done < "$input" #Give while loop an input value

Bash Shell Batch "DNS NXDOMAIN Check" and dirty output

Replace if branch with:
host -t ns "$line" | grep NXDOMAIN

Bash Shell Batch "DNS NXDOMAIN Check" and direct removal of line in file

Replace if branch with:

if host -t ns "$line" | grep NXDOMAIN; then #if domain in line does not exist, i.e. has no DNS records
  sed -i "/^$line/d" "$1" #remove line (remove the line that wholly contains the text of the current line in the file passed as argument) IRREVERSIBLE!
fi