Thanks for voting and commenting on this issue. Your feedback is key to helping us understand how you use JIRA so we can continue improving your experience. We have reviewed this issue over the last few days. We don't currently have any plans to support a temporary "read-only" mode to maintain uptime during upgrades. We expect that in the future, we will concentrate our efforts instead on the ability to perform "hot" or rolling upgrades across nodes in JIRA Data Center.
For customers that are simply looking for the ability to put one or more projects into a "read-only" state, refer to the documentation here.
Please remember that jira.atlassian.com is one of many inputs for the JIRA roadmap. You can learn more about our process here.
I understand that our decision may be disappointing. Please don't hesitate to contact me if you have any questions.
Product Manager, JIRA Platform