To quote the document, KPIs "will be deprecated as alternatives become available". For VFS, no alternative is available, so it's intentionally not deprecated.
They currently offer no replacement for VFS. But I bet they will offer something fuse-like if they offer anything at all. Also: the file provider api they use for iCloud Drive that was supposed to ship in Catalina but got yanked is still likely to happen.
The latest version of FUSE for macOS, which is still freely available to end users but no longer open source and not licensed to allow redistribution, works on the current version of macOS. The open source code has not been updated in a few years and as such only works on older macOS releases.
2 weeks ago I used OSXFUSE to read an old Linux hard disk. It was running ext4 and I was able to install OSXFUSE and EXT4FUSE under Catalina.
It let me browse and copy files off. I had a few directories that were password-protected and I wasn't able to access them. There might be a way to do so but they just weren't important enough to me to do more investigation.
They say that future OS releases will no longer load deprecated KPIs, but they do not note the VFS KPIs as deprecated in their list.