The adjustment brush in develop mode has disappeared in the latest update of Lightroom Classic. The tools I am seeing are completely different from what they were before. It is now showing mask, red eye, healing brush and crop overlay
When you upgrade to a new version of Lightroom Classic, a "What's New" screen appears, and you should always look through it. The masking has changed, it is a major enhancement to the masking tools, and you can read about it here (and see it in action in probably dozens of YouTube videos). The adjustment brush is in there.
DJ_Paige is right - the new MASKING features in LRC are just spectacular and powerful. Best videos I've seen are from 3rd party vendors.... I thought I knew the new masking features well, then I bought a tutorial and was "re educated" even more!!! Defintiely worth learning!!