It hasn't had a meaningful update in ~10 years, and the problem is it still has the brand recognition which keeps potential users away from LibreOffice. It's an embarrassment to Apache if you ask me.
OnlyOffice is nowhere near as full-featured as LO, as well as having huge performance issues especially when dealing with large spreadsheets. I have no idea why it keeps getting recommended.
OnlyOffice is not based on LibreOffice. There might be a point in joining forces with OpenOffice if OpenOffice actually had forces to join with, but it doesn't because it is a dead project.
One of the common problems plaguing Apache is that a lot of their software rots on the vine for official support. OpenOffice is one of them, and it came into ASF like that, because the Oracle buyout caused a lot of Sun projects to wither. See also: Solaris and MySQL, which had very public forks.