I just downloaded LibreOffice_6.3.0_MacOS_x86-64.dmg on a machine with a Catalina Beta (10.15 19A526h). After I install LibreOffice in my Applications and try to run it, I get a warning:
“LibreOffice” can’t be opened because its integrity cannot be verified. This software needs to be updated. Contact the developer for more information.
In general “integrity verification” is a function of MacOS for “non-apple applications” and thus you’d better ask this Apple Inc. But you should be able to cirmumvent the integrity checking by executing:
xattr -d com.apple.quarantine /Applications/LibreOffice.app
(I’d never expect a released software to be supported on a BETA version of an operating system and I cannot image that The Document Foundation would make a statement “LibreOffice is supported on MacOS Catalina Beta”)
Note Use this at your own risk, since it is bypassing what apple considers an important security feature