Hello,
First, I have seen pretty nice changes in the last couple of months in Jira. In my opinion, the interface clearly gets tidier and user-friendlier each month.
One thing that is still quite disturbing: How to remove the "Atlassian project"-box that appears on the right side of every Jira work item (except sub-tasks)?

I know this box is part of "Platform experience" (former: "Atlas"), but we don't want to use it (yet). We have deactivated "Platform experience" but still, we see this useless box.
Why displaying such a box anyway when we already have disabled the entire feature? Users don't have the chance to enable it as they are not admins. If you want to remind admins, ok - but for users this is just cluttering the user interface.
About why we don't enable "Platform experience":
If this box would always display to which project a work item belongs to, it would be of value for us. But such a information is only displayed in the linked work item, not in all its child work items. So the box says "Share weekly updates by linking this Jira work item to a project" in 99% of our work items which makes it useless. Not every work item reflects a project. Only the top-parent (the parent of the parent of the parent of... well, you got it) of the work item does.
(Suggestion for @atlassian: Please hide this box automatically unless the work item belongs to the top-ranked work type in the hierarchy or show to which project the work item belongs to. Best would be: show the parent and the parent of the parent and ... until it reaches the top-ranked work type).