This once-obscure method, that – I guess – is the low-level call used by most of the new Unity GUI … is now the only way of drawing meshes in GUIs. The previous options have been removed, with brief comments telling you to use .SetMesh instead. Half of the DrawMesh / DrawMeshNow methods have also been removed (no explanation given in docs), which were my other go-to approach.
Unfortunately, no-one has documented SetMesh, and it has significant bugs, and breaks with core Unity conventions. This makes it rather difficult to use. Here’s the docs I’ve worked out by trial and error…