I agree with the Glorfindel regarding that there are already several sites where where questions about spreadsheets are on-topic. Having a new site to focus only on spreadsheets, IMO, is possible but there should be a community with specific needs that aren't not safisfied by the current communties. Probably this might be community of users that use spreasheets for complex finance models, business intelligence according to specific industry standards and practices.
Regarding the broad use of spreadsheets...
Stack Overflow (SO) was not mentioned in Glofirdel's answer. I understand why. Questions about the basic use of a spreadsheet are off-topic there, but questions about writing formulas, specially complex formulas, questions about writing VBA macros for Excel and Google Apps Scripts for Google Sheets are on-topic.
Regarding the specific case of Libreoffice SO has more than 800 questions with [libreoffice-calc].
Besides SO there are sites where might questions about spreadsheets, as was mentioned by Glorfindel, might be on topic, but that doesn't meant that those questions will get an answer in any of them.
Super Users looks to have a good user base answering questions about spreadsheet formulas in general. This site has more than 600 questions with [libreoffice-calc].
On Web Applications the top tags is [google-sheets]. Mosts of the questions are about spreadsheet formulas.
On Android Enthusiasts are very few questions with [google-sheets].
On Ask Different (Apple) are [numbers], the spreadsheet app from Apple, there are more thatn 600 questions.