Using a pencil is recommended for the IELTS Listening and Reading of the test.
In many test centres, we now also require the Writing test to be completed in pencil. This is because gradually, all of our test centres are upgrading to new scanners. These particular scanners work best with a pencil as there is no risk of your answers smudging (whereas there is with pen/ink). These test centres are also using answers sheets with a slightly updated design.
No changes have been made to the test format, content, or marking standards. This is simply an operational change. It is also important to note that if you forget to bring a pencil, the test centre staff can provide a pencil for you.