The particular extension is the Alternative dialog Find & Replace for Writer (AltSearch). I have an old reliable Libre 3.6 and wouldn’t mind updating, but extension updates continually trail far behind the current Libre (at this point it is 4.3). How would one download an earlier version of Libre for which the extension is updated to?
All the former versions of LibreOffice are available from Index of /libreoffice/old . I don’t know anything about the extension you seem to need.
Thank you very kindly.