I give up, how do I insert another Writer file into the current file? When I search online, the answer generally is “Insert > File” or “Insert > Document”, but in Writer 6.1, there’s no File or Document option in the Insert menu.
One way is to Insert→Section, and choose [x] Link and select the file.
This works, but is there a way to show the Headers available in the original document in the new document’s section in which it was inserted?
(e.g. consider Doc1.odt has a header “Document 1” and Doc2.odt has a header “Document 2”. When we insert link to these document in Combined.odt – Doc1.odt in Section 1 and Doc2.odt in Section 2, Section 1 should have header “Document 1” and Section 2 should have header “Document 2”. Is this possible?)
@kpk.in: Your comment is a separate question; please file it separately, to have own answers addressing your specific case (and that would require either using different page styles, or having that “Document N” header contents inserted using fields).
In 6.1, it should now read something like: Insert>Paste text from file ...
With a folder icon 
at the beginning.
With me Win-10-1803-64-Bit, LO-Version 6.1.1.2, UI-DE
More a comment than a solution.
I found ebot’s solution after quite a bit of searching - having deliberately not tried Insert>Paste text from file because the file I wanted to include was a writer document that contained images as well as text.
What’s wrong with Insert>Post content from Writer file?
I understand that one might want to import text from any sort of file, and that there are defined OLE object file types, but surely the current case is one that crops up a lot, and could have its own menu item. ??
Sorry, ask.LibreOffice is not a request concert. Questions are asked at ask.LibreOffice, and if you have time and desire, hopefully it will give you a correct answer. If you want to get rid of wishes, you can enter them on Bugzilla. Thank you.
Please do not write a comment as an answer, thanks.