The sidebar needs the diffSidebarOpen setting and a 144-column terminal, and the panel itself is gated off.
What
New code drives a side panel that toggles between a diff view and the conversation, and cycles what the diff is compared against: the session's starting point, uncommitted changes, or the branch. Both the tab switch and the base switch emit telemetry. Opening is gated on the diffSidebarOpen setting, needs a terminal at least 144 columns wide unless that setting is explicitly true, and reports a message when the directory is not a git repository. The chosen base persists as diffSidebarBaseMode.
Evidence
repl_diff_base_switch
Strings lifted out of the shipped bundle, so the claim above can be checked against them.
Names in the bundlediffSidebarOpendiffSidebarBaseMode