I think the answer would be similar to where to ask questions about using MS Office products, like this one: If I had a question with Excel, which site would I use?
It depends on what exactly about LibreOffice do you want to ask.
Is it about general usage (installation, running the software, using its command line parameters, configuring settings, how-to, etc.)?
Is it about general usage on a particular flavor or distribution of Linux?
Is it about automating something with macros or writing code that uses the LibreOffice API?
Is it about writing formulas?
- There is quite a bit of discussion related to whether Excel formulas are on-topic for SO or whether writing spreadsheet formulas are considered as programming. But it seems this is a point of overlap and formula-type questions seem to be accepted on either SO or SU. The same guidelines should also then apply to LibreOffice formulas.
- On SO, the excel tag has this note which can help in deciding on which platform to post on: "Only for questions on programming against Excel objects or files, or complex formula development...General help regarding MS Excel for single worksheet functions is available at Super User"
Is it about asking which plugin or extension is best for something?