I don't think there's too much information about that, as that's the kind of diplomacy that's usually not happening in public.
Though, yes, you would expect that it is your own diplomatic corps that gives you a briefing on what is currently happening in this country and what are touchy subjects. Mostly to prevent you and your ghostwriters from making unintended statements. Idk making references to Winnie-the-Pooh when talking to or about Xi Jinping or something like that. So that when you touch these topics you know the atmosphere in the room and know when you're crossing lines. So that when you do it, you do it with deliberation and have some expectation as to whether that was a minor offense or an "act of war".
It's also possible that the foreign diplomatic staff may drop some hints as to what they want to talk about or not want to talk about or what are topics that are important, what enrages them and what can be calmly discussed. That's most likely very normal.
When it comes to proactive censorship though you would not expect there to be any official physical documents that you could provide as evidence for coercion whether it's towards the people of that country or to the people in your own country. Those most engaged in censorship, would most likely also censor that censorship and pretend that touchy topics simply didn't come up and that their own crimes aren't a topic outside of their country (and thus not a big deal to complain about *wink*).
So what is actually more likely is that Victor Orban wanted to stage a scandal. He's quite infamous for being, well not very democratic (members of his own coalition in the EU called him a dictator) and he's pretty right wing anti-immigration, anti-LGTBQ, pro-Russia aso he's basically anti-EU values except for when they are monetary and moving towards Hungary.
According to the article last years speech featured racist remarks about mixing with non-Europeans with regards to immigration.
As said if he got an OFFICIAL list of topics to talk and not talk about, then it's likely nothing that Romania would be afraid of being leaked and it's more likely that he's taking it out of context. So idk the schedule of the day, some mission statement or declaration of value or something like that.
I mean the "no-go-list" that he mentions includes:
listed as not permitted for discussion were minorities' rights, national symbols, and presenting Western values in a bad light
Which he counters with:
Western values are migration, LGBT [rights], and war. We do not have to put them in a bad light, since they themselves do that
Some of these points are ambiguous as to what they refer or are interpreted to refer to by either side, but for example minority and LGBTQ rights could be something where they legit unapologetically could have stated that using that would be off limits and have it not even be a big deal with most world leaders as it wouldn't necessarily be a point of conflict but rather an affirmation of shared values.
So that sounds like Orban tried to stage a "They don't want me to talk about that, so I'm doing it anyway"-moment. But most of these points, aren't some censorship of topics but rather broad and general values that are between odd and criminal (according to human rights), to disagree with.
There are some point that are ambiguous and without access to that list (though it even exists in the first place) it's hard to tell what it means and what the intended purpose of it was. Though either way that sounds very fishy and more like Orban wanted to present himself as "speaking truth to power" and "being censored by [insert some scapegoat]" and to that end picked something probably inoffensive and made it a prop to that end.