The LibreOffice version 6.3 had been published on August, 8th 2019: https://blog.documentfoundation.org/blog/2019/08/08/tdf-announces-libreoffice-63/
But it seemed nobody took care of an update of the available LibreOffice versions on the LibreOffice extensions website (https://extensions.libreoffice.org/extensions). This new version was added there just recently. Thus the contributors of the LibreOffice extensions website get a notice of the new LibreOffice version and the opportunity to mark their extension as compatible with that version very lately.
But there are some contributors of the LibreOffice extensions website which reacted very quickly and marked their extension compatible with the newest LibreOffice version. I want to thank them for their quick reaction and their contribution to the project.