When using new versionss of LibreOffice with ia2 enabled, it's not possible to now the line number, although announcing this information is enabled in the screen reader, as Libreoffice doesn't send this information to the screen reader. Currently I use NVDA, but in other screen readers, the result is the same. I'm using the 4.2 release, but I have already tested with 4.3 alpha.
Using LibreOffice 18.104.22.168 (x64) and NVDA 2017.3, ensure line numbering is turned on in NVDA:
1. Press NVDA+control+d (insert+control+d by default) to open the document formatting.
2. Press alt+n to toggle line numbers.
3. When on, press ENTER to save changes.
In LibreOffice Writer. Line numbers are read aloud when navigating (eg using the arrow keys) but not when typing (eg when pressing enter to move to a new line). If using a Braille display, line numbers are displayed, although without the number symbol when enter is pressed to move to a new line.
Additionally, if enter is pressed to a new line, then you press say up arrow to navigate away, the line number of the line moved to is not reported.
Issue on NVDA issue tracker 7699: https://github.com/nvaccess/nvda/issues/7699