No European monarchy keeps a ‘court’ in the sense the OP uses it, although one could equally argue that all of them still have them in the sense that they all have formal households (admittedly of varying sizes and formality) and hold court ceremonies. But it’s the decline of the court as a social centre that the OP asks about that’s the interesting point anyway.
The basic shift is easy to pinpoint. What has changed is not that monarchs no longer entertain, as most monarchs still regularly hold receptions, dinners, garden parties etc., just indeed as most presidents do. No, the difference is that those attending those events have to be invited.
Now for the counterintuitive bit. The reason why the classic events associated with old-style courts, such as the levee or the drawing room, could in the past be seen as an important feature of the wider social life of particular capital cities, was that the guests chose to attend them. Even if this required going through a formal application process (and often it did not), it was those being ‘presented’, whether debutantes or anyone else, who took the initiative. Attendance was a statement by the guests, not an honour bestowed by the host. Its key attraction, that it was a semi-public event at which the fashionable could show off to the fashionable and the unfashionable, would have been lost if they had been more restrictive.
In the case of the British monarchy, this system survived into the twentieth century. Levees still had some social caché under Edward VII and, of course, the presentation of debutantes continued until 1958. In the case of some other courts, such as those of Berlin or Vienna, such relatively open events survived as long as the monarchies themselves.
Except that Clarence House is adjacent to and interconnects with St. James’s Palace. So whether they’re the same building can be argued either way. And although Prince Charles did indeed use part of St. James’s before he got Clarence House, most of the building is used as offices for parts of the Royal Household, such as the Lord Chamberlain’s Department and the Royal Collection.
Having been inside St. James’s Palace, I can say that it most certainly isn’t a ruin. It is true that much of it isn’t very grand (which is why it gets used as offices), but, even then, there’s an important exception - the sequence of state rooms along the Mall front climaxing with the Throne Room is, if anything, more impressive than the equivalent sequence in Buckingham Palace. But then that’s because it was specifically designed for holding levees.