The Raspberry Pi OS (formerly Raspbian) officially moved away from using raspistill as the default command-line tool for camera capture starting in Raspberry Pi OS Bullseye, which was released in October 2021.
raspistill was part of the raspberrypi-userland package, relying on the Broadcom-specific GPU libraries (such as MMAL and bcm2835-v4l2). However, with the release of Raspberry Pi OS Bullseye, the Raspberry Pi Foundation transitioned to the libcamera stack for handling camera operations, which is more modern and cross-platform.
The libcamera framework replaces older tools like raspistill and raspivid with new commands that are more flexible and can support a wider variety of cameras beyond the Raspberry Pi's own camera modules. The equivalent to raspistill in the new system is the libcamera-still command.
So, to summarize:
Raspberry Pi OS Bookworm renamed the camera capture applications from libcamera-* to rpicam-*. Symbolic links allow users to use the old names for now. Adopt the new application names as soon as possible. Raspberry Pi OS versions prior to Bookworm still use the libcamera-* name.
To find the version of the Raspberry Pi Operating System your Raspberry Pi is running use the following Command Line command: