You can find the changelog at https://wordpress.org/support/wordpress-version/version-5-5/
Regarding any problems, please see the known issue and common fixes at https://wordpress.org/support/topic/read-this-first-wordpress-5-5-master-list/ first.
If I understand correctly, there were no security fixes in the new version, so I can go back to the previous version until the plugin and theme developers fix their compatibility bugs.
you can return to the test from 5.4.2, or update the wp-admin + wp-content folder and the wp-settings.php file overlaps your current web directory, or you can delete it to copy it again. to normal operation again.
If I understand correctly, there were no security fixes in the new version
Correct, and if there were, they would have been backported to the 5.4 and earlier branches anyway.
Does this mean that the owners of the sites of the earlier versions of the WP are also safe and I can no longer update if I am completely satisfied with the current (or previous) version? How are security changes rolled over to older branches if auto-update is disabled?
How are security changes rolled over to older branches if auto-update is disabled?
You will need auto-update enabled, otherwise hitting the update button will bring you up to 5.5.x instead of applying the latest 5.4.x security update.
Alternatively, update manually: https://wordpress.org/support/article/upgrading-wordpress-extended-instructions/
Thanks for your support James @macmanx.
So, at the moment I am using the plugin WP Downgrade back to version 5.4.2.
If I uninstall this plugin and keep the current version 5.4.2, then I assume it will automatically update to 5.5.
How can I keep version 5.4.2 and still use automatic updates for that version?
Major versions do not auto-update, 5.4.2 won’t automatically move to 5.5, you’d have to manually hit the update button.
The only things that auto-update are minor versions, so 5.4.2 will auto-update to 5.4.3 whenever that’s released.
I am very grateful to you for your answers @macmanx.