SIP is disabled, but /usr/bin is write protected anyway (macOS Catalina beta 10.5)

Catalina has a new file system arrangement where most of the system files are write-protected. Apple uses two partitions, a read-only one for the main system files and a writeable one for other files, and melds them together and presents them as one to the user.

For details see the WWDC video What's New in Apple Filesystems

I assume this answer might be relatively off-topic, as I'm not running the beta version of Catalina, but the official one.

In order to perform changes to /usr/bin, you'll have to perform the following steps.

1. Disable System Integrity Protection (SIP).
2. Mount / as writeable.

Disable SIP

• Reboot your system into recovery mode (reboot and hold cmd + R while booting)
• Use the appropriate credentials when prompted.
• Navigate to Utilities -> Terminal in the top bar.
• In the terminal session, execute # csrutil disable.
• Reboot into normal boot mode (# reboot).

Mount / as writeable

With the OS in normal mode, start a new terminal session and execute the following:

• $ sudo mount -uw /

Please note that / will be mounted as read-only on the next system boot.

In order to enable SIP, the process is exactly the same as disabling it, except for the fact that you would have to run # csrutil enable.