The situation has eased. The economy is still in the shitter due to mismanagement (prejected growthgof 1.9% for the year) but a lot better than the dark days of 2015/2016.
The reason for the restriction was that, for years, Nigeria had supported its currency with the use of its foreign reserves. The price of oil made that somewhat manageable. When oil crashed in 2015, it became difficult. The country burnt through a third of it's reserves in a little over six months. The new government felt it had little choice but to change to a free float currency regime. The £ went from about ₦200 to ₦600 even more on the black market. So the government imposed currency restrictions and import controls. Around that period, it took my parents 3 months to purchase $10,000
Now the £ is about 450 Price of oil going back up has contributed to that. There are still some import controls, especially agricultural. Some capital controls are still in place as well. But these are mostly for individuals not businesses.
The sad part is a lot of business that depended on forex went bust during that period.