We don’t have to erect a border , they won’t demand it that’s absolutely true but you know that any member can then make a complaint , or rather raise a dispute, under the grounds that the lack of a border grants the republic .
Here’s a fairly comprehensive text on it , the last line might be of particular interest to you .
There is no rule in the WTO requiring its member governments to secure their borders.
The WTO does not tell countries what to doother than to keep their promises (abide by the WTO agreements and their WTO commitments)
Even when countries break their WTO promises, there is no “confrontation” with “the WTO” and least of all with “WTO officials”
The WTO is member-driven. If in the future other WTO countries believe the UK is violating an agreement, it is they, not the WTO bureaucracy, who will act. They can do so by complaining in a WTO meeting or filing a legal challenge in WTO dispute settlement
Since there is no WTO rule requiring governments to secure their borders, failing to do so would not break any specific agreement
Where the UK might run into trouble is under the WTO’s non-discrimination rules, particularly “most-favoured-nation” treatment (MFN), which means treating one’s trading partners equally
Suppose the UK and EU trade on WTO terms after Brexit. Suppose American apples arriving in the UK at an English port have to go through controls, but Irish apples crossing the border into Northern Ireland (also the UK) do not. Then the US could complain that its apples were discriminated against. They weren’t given equal treatment with Irish apples when they entered the UK.
The US might seek a legal ruling in WTO dispute settlement. Months or years later, the ruling might conclude that the UK had discriminated. So either checks at the English ports would have to be dropped, OR CHECKS AT THE IRISH BORDER WOULD HAVE TO BE SET UP .