Parler Pitched Itself as Twitter Without Rules. Not Anymore, Apple and Google Said.

In the eyes of many of Mr. Trump’s supporters, Parler was a safe haven from so-called Big Tech censorship — a place where they could espouse conspiracy theories, make threats and even plan violent rallies without worrying about getting banned. It had been one of the most downloaded apps in recent months just as Facebook, Twitter and Instagram increasingly cracked down on hate speech and misinformation.

But it is now clear that Parler will not be able to maintain its free-for-all status if it wants to be able to keep its wide reach. Apple and Google make the operating systems that back nearly every smartphone in the world, and they roughly split the market in the United States.

If Apple pulls Parler from the App Store, people would not be able to download the app to their iPhones or iPads. People who had already downloaded the Parler iPhone app would still be able to use it, but the company would not be able to update the app, meaning it would eventually be rendered obsolete as Apple updated the iPhone software.

Google’s suspension is problematic for Parler, but people with Android devices will still be able to get the app, just with a bit more work. Google allows other app marketplaces on Android, and its decision applies only to its flagship Play Store.

And people will also still be able to use Parler via web browsers on their phones or computers.

Parler’s app has been downloaded more than 10 million times on iPhones and Android devices, with more than 80 percent of the downloads in the United States, according to Sensor Tower, an app data firm. On Thursday, the day after the riot in Washington, people downloaded Parler 39,000 times, more than twice as much as the day prior.

That success had put Parler in the pole position among a number of social-media upstarts trying to attract people on the far right who were fed up with the actions of the tech companies or who had been banned themselves from the mainstream platforms.

Parler quickly drew some of the biggest names. Upon signing up for an account, Parler prompted new users to follow people like Sean Hannity, the Fox News pundit, and Phil Robertson, star of the reality television series “Duck Dynasty.”