Can I export all backlog items in a backlog or an entire project?

Yes, here's how you can export all backlog items in a backlog:

1. Go to the Backlog page.

2. Expand all backlog lists in the backlog, for example by right-clicking on the name of a backlog list and selecting "Expand all backlog lists".

3. In the "Show" dropdown in the lower right corner of the page, select "Filtered items".

4. If there are any filter criteria in the current filter, click "Reset this filter".

5. In the filter, add a "Status" filter criterion where you select all statuses, like this:

So that it becomes a "Has any status" filter criterion, like this:

6. Press Ctrl+A / Cmd + A to select all matching backlog items (which is all backlog items in the backlog).

7. Right-click while over a backlog item and select "Export these items".

8. Click "Save the file".

Note that for epics, this will include both the epic and all child backlog items in it, so be careful to not count the work in those twice. To exclude the epics, but include their child backlog items, add an additional "Type" filter criterion where you select all other backlog item types than "Epic", like this:

You can also add other filter criteria to further narrow down which backlog items to export.

If your project has multiple backlogs, do an export like this in each backlog.