The black and white international country identifier is covered by international law and has been in effect since at least 1919. All signatories to the International Conventions on Road Traffic have to accept it.By default a non EU country has display the black and white country sticker so that as such would not have to be explicitly written. What is interesting is that the government has told the plate makers they cannot produce a plate with the blue background stars with GB in the middle. However you can buy a plate with an area that has stars and GB as long as it’s background is not blue. It’s a strange weird world and on another thread regarding pet passports people are happy to pay for a EU pet passports to circumnavigate the system. ( this is not pro or anti Brexit just observing the bizzare nature of it all).
The euro-stars and country identifier on a number plate is an internal EU agreement between member states (and some EEA nations) that they will not require the international convention identifier to be displayed on vehicles registered in their member states if the "europlate" is fitted to the vehicle.
UK law sets out the country identifiers that are allowed to be displayed on UK number plates. Some, such as those for England, Scotland, Ireland and Wales are internal and only recognised within the UK - they have no legal standing internationally. "UK" is also permitted by UK legislation but not recognised outside of our borders.
The EU stars and GB identifier were of course permitted to be displayed on UK number plates when we were an EU member state. It was always voluntary, but some EU states have made the "europlate" mandatory for vehicles registered in their country.
Now we have left the EU, we cannot avail ourselves of the "europlate" as we are no longer a member state. So it is no longer legal for UK registered number plate suppliers to include it on new number plates. The EU would also have been within their rights to insist that it must no longer be displayed on any UK registered vehicles circulating with the EU, but common sense has prevailed in that existing europlates do not have to be removed from UK registered vehicles; but as they are no longer recognised the International Convention sticker or plate must be separately displayed, as would be the case anywhere else in the world.
Except that, an agreement appears to have been reached with most of the EU (Spain, Cyprus and Malta excepted) that the Union flag and/or GB identifier alone on a UK number plate (which were previously legal in the UK but not recognised anywhere outside of our borders) will be acceptable instead of the International Convention sticker.
None of that prevents any driver from displaying an International Convention sticker instead of, or as well as a recognised and acceptable "GB" number plate if they wish to.
You can buy number plates with all sorts of things on them that are not allowed, from all sorts of internet businesses, market stalls and other "number plate suppliers" who refer to them as "show plates". EU stars on a black background would fall into that category - it is not legal anywhere. There is no such thing as a show plate in law, and UK legislation actually states that any number plate that does not conform to the legal requirements, or any item that is made up to appear to be a number plate, is actually a "counterfeit plate", the display of which can be subject to various penalties, whether on or off road.