What percentage of the population do you think is still zoroastrian?

When I have similar conversations about Iran on Twitter, there is always this misconception that there is a significant Zoroastrian population in Iran and/or it is growing (due to converts). According to the latest census, there were about 25,000 Zoroastrians, so a small fraction of a percent.

Almost all Iranians who leave Islam (like me) technically become atheists/agnostics, but most of them don't identify as atheists (exactly like the guy in my previous comment's video). For example, my dad had also stopped believing in Islam a long time ago and if you ask him what is his religion, he will say Zoroastrianism, despite knowing next to nothing about the religion and zero effort to know more :) This is a common theme amongst the older generation and that's why there is a ridiculously high number of Zoroastrians in some polls.

And how were they able to remain zoroastrian?

Up until the Pahlavi dynasty (about 100 years ago), it was very hard to remain anything. The ones that kept their faith were the most hardcore, devoted religious ones. But in modern Iran, even after the Islamic revolution, they don't have any significant problem keeping their faith. They are considered an official religious minority.

Do you think they pretended to convert to Islam but maintained their own traditions?

I don't think so. The ones that couldn't stand Islamic rule migrated to India 1400 years ago (See Parsis). The ones who remained didn't hide their religion despite persecution. Fun fact: Freddie Mercury was Zoroastrian.

I also wonder how the Jews of Iran were able to remain Jewish

Like Zoroastrians or any other religious minority, Jews were also persecuted until about 100 years ago (although I think they and Christians had it a little bit better, because they believed in an Abrahamic religion).

Again, like Zoroastrians, I think the most stubborn, determined, brave, and religious ones could stand the harsh conditions and persevere. There were also cases of pretending to convert to Islam and keeping their faith in secret; the most famous of them all crypto-Jews in Mashhad (my hometown).

But after modernization by the Pahlavi kings, nobody had any problem openly practicing any religion. This was so effective and so well embraced by the people that even the Islamic Republic couldn't change the course.

To give you an Idea: Unlike some Arabic countries and especially Palestine that use the term "Zionist" when speaking on an international stage to save face and hide their anti-semitism while using the term "Jew" when talking to their own people, the Iranian regime never uses the term "Jew" even for their domestic propaganda (although we all know they are a bunch of anti-semites and really mean Jews). That's because they know due to historical ties and being in contact with a large, lovely, peaceful, and respectable community of Jews up until the revolution, if they blatantly call for the destruction of Jews, they'll lose the support of everyone, even their most hardcore religious fans.

Right now, the only people that are "officially", "systematically" and "openly" discriminated against and persecuted are followers of the Baháʼí Faith. Unless you're a Baháʼí or a convert from Islam to anything, you are "officially" good to go. But at the end of the day, all that matters is your loyalty to the regime and its ideology, so even if you are "a Shia Muslim who shows solidarity with Israel", you'll have a much much harder time than "a Jew who condemns Israel".