Hi Damien 
.
Discourse and Ember only deprecate things once there is an alternative available for immediate use. So if there are any changes you need to make, those can be made straight away and will work under Ember 3.
This is the approach we’re taking in Discourse core and in all our official themes and plugins. We’re not creating separate code for Ember 3 / Ember 5.
I’m afraid it’s not optional. Keeping Discourse core’s dependencies up-to-date is essential for us in terms of security, performance and features. The features which Ember 5 provides are going to become essential for future improvements to Discourse itself.
That is a good question! We’re putting a lot of work at the moment into making this transition as smooth as possible, and are working on various tooling improvements.
One of those improvements landed a few hours ago, and should help you here.
https://github.com/discourse/discourse/pull/24870
If you visit /safe-mode on your forum, there is now a checkbox for “Make Javascript deprecations raise an error”.
That will allow you to safely see how your site works with all pending deprecations converted into errors.
Technically that is not quite the same as “Testing ember 5 on the forum”. But it’s a good way to simulate the upcoming breaking changes without needing a fully separate staging environment.