The term axis was to describe the opposite of the allies - basically the enemy. The Japanese were also referred to as the Axis, and they are not portrayed like the Germans are. The “Axis of Evil” was a term used to generally describe enemies of the Allies when the war started.
Since the term has no real meaning other than being used to identify the ‘enemy’, there should be no reason that the terminology alone is against the terms of service.
Many other game developers have used the term with no problem as it has no real significance when it comes to any form of hate speech or war crimes or the like.
The only thing you have to worry about is the portrayal of the Germans - because many of their icons are what is targeted by moderation. Swastikas, red armbands, and the Nazi Eagle are the primary symbols that are banned and can get you in trouble. The German Iron Cross image is an icon that has often been considered by the general public to be a suitable and appropriate replacement for the swastika in modern culture; many movies and video games will use that icon in place of a swastika to avoid offending the public, or representing nazi political goals.
Of course I can’t comment on roblox’s depiction of the iron cross.