It's a period piece obviously so a lot is not technically eastereggs but just plot. Like how Margaret gets a pamphlet by Margaret Sanger, about how to do birth control by douching with Lysol after sex. Then later Margaret is confronted with a woman who has a miscarriage. She had drank ecoli-infested milk to induce an abortion. Her husband kept assaulting her and she had no choice as they already had several kids. That's based on a real woman who drank ecoli milk by mistake and had a miscarriage. That's what partly inspired Sanger to get into reproduction rights. So the show takes real stuff and integrates it into the plot, like any historical piece does.

There's also cameos and brief shots of stuff that are more subtle. Gaston Means shows up in S3. There's tons of Irish Republican stuff and the war with England. Black and Tans, all of that. There's also a bit about Cuba under Machado. Tons of stuff.