Huh, that's a wide field, as we say in German.

The topic with plates for classic cars in Germany is -for plate freaks and lovers of authenticy- a pure annoyance since 25 years.

Back then the H-plate, also the red 07-plate for classic car collections were introduced.
It was based on tax reasons (special insurances existed before) and was developed by the DEUVET, the association of German classic car clubs together with an engaged Federal Minister of Transportation (nearly the only, who wasn't a moron).

Already back then I warned, that a special plate would be the death for authentic plates (introduced on 1.7.1956) on old cars - but I was told: "Keep quiet. It's a topic of EU-wide harmonisation" - but this was pure bullshit.
Back then Germany was the only EU-country with a special plate for historic cars. Many years later only Poland and Spain followed.

In several EU-countries as Holland, Denmark, Luxemburg, Britain and IIRC also France you can have plates in the old style for classics.
No, the reason was annother: it was a lobbyism-thing of the monopolist (in many countries worldwide, not only in Germany) maker of license plates.