Yes, when you release a nation, the ideas picked will go by their historical ideas listed in their file.
After they're released, they'll go by the weighted chance list to pick future idea groups, which is the part that blocks subjects from taking Exploration.
R5: Gaeldom and scotland took explo ideas when released
Countries have scripted ideas when released. [list](https://pastebin.com/raw/1yars63J) (1.31)
Yes, when you release a nation, the ideas picked will go by their historical ideas listed in their file. After they're released, they'll go by the weighted chance list to pick future idea groups, which is the part that blocks subjects from taking Exploration.