Should I turn off the Backward Compatibility plugin?
Only after everything works without it. It keeps older extensions running; turn it off on a copy of your site first and test.
What the plugin does
After a major upgrade, Joomla keeps older extensions running through a compatibility plugin: Behaviour - Backward Compatibility on Joomla 5, and Behaviour - Backward Compatibility 6 on Joomla 6. It maps old class names to the new ones.
When to turn it off
- Only when every extension and your template say they are built for the current version.
- Turn it off on a test copy first and click through the site and the administrator.
- If everything works, turn it off on the live site, with database access at hand.
Before upgrading 5 → 6
The Joomla 5 plugin (Behaviour - Backward Compatibility, without a number) must be off before you start. Joomla turns on Backward Compatibility 6 for you during the upgrade; the two can't be on together.
Locked out after turning it off?
Turn it back on in the database: in the #__extensions table, set enabled to 1 for the plugin (plg_behaviour_compat6 on Joomla 6).
Good to know
- Joomla 7 will drop support for extensions that still need the plugin, so plan replacements during the Joomla 6 years.
Sources
- Joomla 5 to 6 Planning and Upgrade Step by Step – Joomla User Guide
- Joomla 4 to 5 Planning and Upgrade Step by Step – Joomla User Guide
