Softbank's announcement says that as of Monday, September 5th, it owned “all of ARM’s issued and to be issued shares”. Today, Tuesday September 6th, “ARM will be delisted from the London Stock Exchange … and will cease to be a listed company.”
The Japanese company will now roll up ARM's results into its own.
Now, to deal with Brexit, which threatens to make it tricky for SoftBank to keep EU citizens on its UK payroll. Skilled silicon-wranglers are not easy to find, so SoftBank will doubtless be interested in Japan's Brexit wish-list as it calls for current skilled migration rules to just about remain intact. ®