This post was last updated on 4/08/21.
Update from April, 2021: There are fewer and fewer reasons to use Salesforce Classic anymore. Lightning is much closer to feature parity with Classic than it was when this blog post was first published in late 2019. Additionally, many new Salesforce features are built strictly for Lightning experiences. In fact, Salesforce is removing end-user access to Classic by default in some new editions.
If you feel strongly that you want to continue using Salesforce Classic, you need to log a support case with a business reason.
In this post, we’ll outline how to quickly opt out of the auto-transition that Salesforce is pushing for October of 2019.
To be clear, Salesforce is not sunsetting Classic anytime soon. There are still too many orgs that depend on the functionality and UI of Classic, and requiring everyone to switch to Lightning would be a bad move. Instead, Salesforce asks that you submit a ticket in order to opt out of the switch. However, if you do submit this ticket, Salesforce simply sends you a link to a URL that leads to a page that asks why you’re opting out, and then prompts you to choose a target date to start using Lightning.
To opt your org out of the Lightning Auto Transition, first open this link:
https://lightning-readiness-check.salesforce.com/autotransitiondelay
If you also need to stop auto-transition emails regarding Lightning upgrades:
You have successfully delayed your Lightning auto-transition for your Organization.