Koeman secured qualification with ease, successfully rejuvenating the Dutch, who had missed out on the last European Championship and World Cup.
They finished second in their qualifying group, behind Germany, but scored plenty of goals and dropped just five points.
Koeman’s side also performed well in the Nations League, reaching the final, where they lost 1-0 to Portugal.
After he stepped down to take the Barcelona job, De Boer was recruited as his replacement, having recently left Atlanta United.
The former Netherlands defender, who won 112 caps for his country during an illustrious career, started out as a manager with his first club Ajax in 2010.
He won four Eredivisie titles over the next six years, to add to the five he won as a player in the course of a hugely successful period at home and abroad.
De Boer left with his status as an Ajax legend enhanced, but ran into difficulties with brief and disastrous spells at Inter Milan and Crystal Palace.
He lasted just five games, four of which ended in defeat, at Selhurst Park, before being sacked and replaced by Roy Hodgson.
Heading to America with Atlanta was a chance to rebuild his reputation, which started well with two trophies in his first season, but ended with an early exit from the MLS is Back Tournament.
De Boer has made a patchy start as Netherlands manager, winning just four of his first nine games, ahead of the real test at Euro 2020.
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