From Joomla! Documentation
New to Joomla! 3.10.x is the Pre-Update Check. This view of the Joomla Update component shows technical specifications of the server the site is on and core and third-party extensions that use the Update Server in a list form.
The top portion of the Pre-Update Check shows whether your current server environment is compatible with the current target version Joomla for both the required PHP and database settings. The data on the Technical Requirements page show the specs we are checking against.
What's important is to check if anything is red and not compatible with Joomla 4.x. If it isn't, you'll need to talk to your host or change hosts before migrating to Joomla 4.x. During a migration is a great time to change hosts. See the Migration Funnel for more information on planning and step-by-step instructions.
Extensions Pre-Update Check
The extension portion of the Pre-Update Check component pulls third-party extensions.
The list is broken into sections based on the targetplatform tag, the extensions you are or are not using. For developers, see Deploying an Update Server for more information about these tags and how to deploy an Update Server. For more information about the Joomla Update System, see the Joomla! Update System Requirement and Extensions Extension Manager Update pages.
By clicking the More Detail link to the right in each colour heading, you can see additional information about the extensions installed on your site.
The extensions listed here do not support yet the Joomla Update system or atleast do not have an targetplatform tag that indicates support for the selected target Joomla Version. (see Deploying an Update Server) Please contact the developer of the listed extensions to make sure they are compatible with 4.x
The extensions listed here require an update to work correctly on 4.x. Please contact the developer of the listed extensions to make sure whether you have to update the component before or after the upgrade.
The extensions listed here claim to be 4.x compatible and no update is required.
What is it about that Potential Upgrade Issue
Plugins with the type ('system', 'user', 'authentication', 'actionlog', 'twofactorauth') could be triggered while we are processing the upgrade. When there is a critical issue within that plugins that could result into a broken upgrade and make the site unusable. For that reason any plugin from that type that is not explicite listed as compatible has to be even more carefully reviewed before you hit the upgrade button. It is strongly recommended to disable the plugins in question and to check with the original developer.
On the Live Update Tab you will be preseted again with the full list of potential breaking extensions.
It is strongly recommended to disable all extensions including the plugins listed here before you run the upgrade because the extensions listed here seem not to be compatible with your target version but could break your upgrade.
You know what you are doing? You want to choose to ignore the warnings and run the upgrade anyway you can do so by checking the checkbox. As that is taking your site at risk of breaking without an easy recovery this is the last time to remind you to take a backup!
Questions and Answers
Let's break this down into some questions and answers.
Q: What do the tags mean?
A: The green No Update Required tag means that the extension has been tagged by the developer that the extension as ready for Joomla 4. It should migrate with the one-click from 3.10.x to 4.x. Verifying with the developer is suggested.
The red Update Information Unavailable tag means that the extension has not added compatibility information the extension. Check with the developer on the migration path from 3.10.x to 4.x.
The yellow/orange Yes (x.x.x) tag means that the extension has been tagged by the developer that the extension may need an update in order to be compatible with Joomla 4. Update the extension and see if it turns to a green Yes. If it doesn't, check with the developer on the migration path from 3.10.x to 4.x.
The gray Missing Compatibility Tag means that the extension doesn't have a tag or does not use the Update Server. The developer hasn't said yes or no or needs an update. In some cases, a package may have been installed and the component portion has a tag but the additional plugins or modules are missing tags. As always, you'll need to check with each extension developer to verify the migration path from 3.10.x to 4.x.
Q: What about the Joomla core components?
A: Joomla core extensions will migrate from 3.10.x to 4.x on the one-click with these exceptions:
The language packs are not yet ready for Joomla 4 (as of September 2020). Over time, they will be. If your site depends on other language packs, wait to migrate until all the language packs are ready.
The Weblinks component is not yet ready for Joomla 4 (as of September 2020). If you're going to continue using Weblinks in your Joomla 4 site, wait to migrate until the Weblinks component is ready.
Q: What about templates?
A: Because of the changes in Joomla 4, your template may or may not be compatible with Joomla 4. The core templates like Protostar, Beez3, Beez5, and so on will convert to the new Cassiopeia template on the one click and the older templates will be completely removed. If you're using a club template from a third-party developer that is not compatible with Joomla 4, you'll need to change your default template to Protostar (or Beez3) before you do the one-click. If you have older administrator templates or core templates, it's recommended to uninstall all of them except whichever of the core templates you will set as default. This includes Bluestork, Hathor, the other beez templates. You will keep Isis as the administrator template. You will keep either your compatible third-party template, Protostar or Beez3 as default. You will uninstall the rest.
Q: Can I uninstall plg_content_geshi?
A: Yes. This is leftover from the 2.5 life cycle and must be uninstalled.
Q: What about all that FOF, fef, etc.?
A: Many extensions in the red or yellow/orange headings will/may make it through the one-click and migrate fine. It is recommended that you go to Extensions→Manage→Manage and check all extensions. Typically the ones authored by Joomla will migrate on the one click. Typically the Akeeba extensions will too. Again, check with extension developers to verify the migration path on all third-party extensions.
Q: Does the Extensions Pre-Update Check show all the extensions installed on my site?
A: The Pre-Update Check will show all extensions. Only extensions that use the Update Server are given compatibility information. Those extensions that do not use the Update Server will show in the list with the Missing Compatibility Tag.
From a migration perspective, it is my belief that the Pre-Update Check should be used a guide. You'll want to have a second tab open with Extensions→Manage→Manage open in order to uninstall any extensions or check who they're authored by so you can contact third-party extension developers on the migration path they've established to migrate their extensions.