In a DR situation when you have lost your entire Qlik Sense environment ideally you would be restoring from your backups using Repository Snapshot Manager. This will restore the repository database which contains all the security rules. If you have not lost the entire site and just want to recover a single security rule and it is one of the default security rules, you could just copy the definition of the rule from another installation of Qlik Sense. If it is not a default rule and you can't remember the definition, or haven't documented it anywhere, it should be possible to retrieve the deleted rule from the PostgresSQL repository database. If you are not familiar with this method, I suggest you contact support for assistance.
We have additional environments with full copies of all the rules so that we can test rule changes before applying them to our production environment. If you do not have the luxury of additional environments and you just want to store a copy of all your rules somewhere and you don't have access to the API's or the database then you could try the procedure I documented here --> Exporting lists from QMC to Excel
I hope this helps.