This is a must have feature as once the user limit reaches entire instance become un-usable. Causing organization wide downtime. As hundreds of teams using and directly impacting deliverables.

Feature should be configurable. Create at least email notification upon reaching custom thresholds.

Refer to the link - https://community.atlassian.com/t5/Jira-questions/Jira-How-to-generate-email-notification-when-you-near-your/qaq-p/895614 shared by Samantha for automating through Custom Script

Jira does not offer a default solution to this. But you can implement a custom script. 

First you can check your active user count from your db with the sql provided here:

https://confluence.atlassian.com/jirakb/how-to-get-a-list-of-active-users-counting-towards-the-jira-application-license-278695452.html

Then you can write a script that compares the count of your user number to your license limit and sends an e-mail if the former is bigger than the later

this is a must have feature which will prevent Jira to be in read only and avoid disruption of service.

A email notification based on a threshold is the good and simple solution or even an alert in slack

I would love an email notification with a customizable threshold to trigger it. Reaching the limit can be too blocking and it surprises me that this isn't implemented yet!

Implement basic email notification that will allow administrators to receive emails when defined threshold is reached i.e 100 licenses away from allowed/licensed users - fire up email.