I discovered something after struggling with a similar issue where the git graph button in the source control panel went missing. In VSCode user settings, there is an option called SCM: Provider Count Badge. This was set to none.

The setting says "Controls the count badges on Source Control Provision headers. These headers appear in the Source Control view when there is more than one provider or when the SCM: Always Show Repositories setting is enabled, and in the Source Control Repositories view."

When I changed this from "None" to "Auto", the Git-Graph button showed up again within the source control menu to the left of the three dot 'more actions..." button.

Took me forever to figure this out so hopefully this helps someone out.