From the 6.0 release notes:
Now extension installation and removal via the Extension Manager dialog (Tools ▸ Extension Manager...) can be dynamically enabled/disabled from the expert configurations (Tools ▸ Options ▸ Advanced ▸ Open Expert Configurations). tdf#36019 (Muhammet Kara, Pardus; Heiko Tietze)
* This also affects the installation by double-clicking the extension file in the file manager, but doesn't affect the unopkg commands.
* When ExtensionSecurity::DisableExtensionRemoval is set in expert configurations, context menu entry of 'remove' for installed extensions is also disabled along with the "Remove" button on the dialog.
These keys have a 'false' value as default:
After setting them to 'true', an application restart might be needed to make it fully effective.
Olivier Hallot committed a patch related to this issue.
It has been pushed to "master":
tdf#114240 Add note on extension security