We do not call it "Niderlandy".

We call it Królestwo Niderlandów.

There is a difference between Wielka Brytania and Zjednoczone Królestwo Wielkiej Brytanii i Irlandii Północnej. And there is a difference between Holandia and Królestwo Niderlandów.

And what with "since 2019"?

Here is the edition of the official list of the Polish geographical names of the world (2019):

• The word "Niderlandy" does not exist in it.
• Geographical name (common name): Holandia
• Official name (full name): Królestwo Niderlandów
• Official language: niderlandzki 'Dutch', also locally zachodniofryzyjski 'West Frisian'
• Administrative units, Brabancja Północna (Noord-Brabant), Fryzja (Fryslân), Geldria (Gelderland), Holandia Południowa (Zuid-Holland), Holandia Północna (Noord-Holland), Limburgia (Limburg), Zelandia (Zeeland)
• Lands, regions:
  • Brabancja (Brabant) also in Belgium
  • Flandria (Vlaanderen) also in Belgium and France
  • Fryzja (Friesland) also in Denmark and Germany
  • Fryzja Zachodnia (West Friesland)
  • Holandia (Holland)
  • Limburgia (Limburg) also in Belgium,
  • Średniogórze Środkowoeuropejskie (Middelgebergte) also in Belgium, France, Luxembourg, Germany and Switzerland

It's hard to grasp why we call them Holadia /s. It's not like some countries (Poland, Germany) are called by ONE of their tribes ;) [I looove how they are offended when we call them by ONE of their regions ;) ]

And THEY ARE NOT Niderlandy) 'Low Countries' in Polish language! Calling them Królestwo Niderlandów is like calling Stany Zjednoczone Ameryki (the US of A): Ameryka ;) The US is not a full "America" - but we call them like that because they call themselves like that. The same with the Kingdom of the Netherlands.

And how they are even "low lands" from a Polish perspective? No one will be translating their name into Polish as such! Belgium and Luxembourg have higher average elevation than Poland (incl. our "high" mountains) and there are also Baltic states ;)

And I do not even want to ask, how Slavic countries that translated the name, call people from the Netherlands ;) Nizoziemiec, it's simillar to niziołek 'halfling, hobbit' ;)

So no, calling the Netherlands as Niderlandy in Poland does not happen (unless by poorly educated people with a strong sentiment for "political correctness").