With his $44 billion acquisition of $Twitter(TWTR.US)$, Elon Musk is betting he'll be able to steer away from the increasing content controls that major social-media platforms have adopted in recent years -- and that users and advertisers will want to come along for the ride.

While his other plans for Twitter aren't publicly known, Mr. Musk has signaled his intention to reduce moderation efforts and embrace more free expression. He is, in effect, talking about returning the platform to its early days as a company a decade or more ago, when Twitter executives publicly described it as "the free speech wing of the free speech party."

In the years since, Twitter and other mainstream social-media platforms have imposed policies and practices designed to make them less susceptible to harassment, manipulation and falsehoods they see as harmful. Those measures aim to make the services more appealing to advertisers seeking a safe venue, and to better accommodate politicians and regulators arguing that the platforms are responsible for all their content. But the moves have alienated some users who, like Mr. Musk, say they go overboard.

How Twitter operates won't be entirely up to Mr. Musk. The platforms are responding in part to pressure from activists, governments and others. On Tuesday, one of the European Union's top regulators warned Mr. Musk that Twitter under his ownership would need to comply with the bloc's rules for moderating illegal and harmful content. "Be it cars or social media, any company operating in Europe needs to comply with our rules--regardless of their shareholding," tweeted Thierry Breton, the EU's commissioner for the internal market.

Mr. Musk's changes, and the response, will be closely watched. Twitter is smaller and less profitable than other social-media companies. It claims 217 million so-called monetizable daily users compared with 1.9 billion daily users for Meta Platforms Inc.'s core Facebook platform and a billion-plus monthly users for TikTok. But Twitter users include media personalities, politicians, activists and tech aficionados who help shape social discourse.

"It has influence way beyond its user base and way beyond its valuation, and so we care about it," said David Kaye, former United Nations special rapporteur focused on free speech and now a law professor at the University of California, Irvine.

Mr. Musk, chief executive of $Tesla(TSLA.US)$ and Space Exploration Technologies Corp. as well as the world's richest person, is one of Twitter's most prominent users, employing the platform for everything from promoting his companies to battling rivals and critics. In Monday's deal announcement, he said he wants to open up Twitter's algorithms to public scrutiny, "defeat" spammers and authenticate users' identities.

Some industry executives and analysts say Mr. Musk is in for an awakening. Reversing Twitter's efforts risks stoking advertisers' worries about the placement of their ads, potentially pushing them toward competitors like Snap Inc. and Meta, said Ben Black, co-head of internet research at Deutsche Bank.

"Do you really want to have your Mercedes-Benz commercial against a beheading?" he said. "If someone falls overboard on a cruise ship, do you want your vacation message being advertised against that content?" He said less moderation could also drive away some users, potentially slowing growth.

In the company's early years, Twitter executives were loath to remove content unless it broke local laws. The platform took often took a hands-off approach even to content from officially designated terrorist groups, drawing criticism from researchers, lawmakers and victims' families.

It stepped up moderation over the years, removing accounts connected to the terrorist group ISIS in 2014, for example. That accelerated during and after the 2016 U.S. presidential race, which saw a jump in complaints about white nationalists and others using the platform to attack and harass minority users.

Other industry leaders who favored looser policies, including $Meta Platforms(FB.US)$ co-founder Mark Zuckerberg, have gradually embraced more intervention. Platforms including Meta's Facebook and Instagram, TikTok, $Pinterest(PINS.US)$ and Reddit Inc. all have added staff or bolstered rules to monitor their content, though each draws the lines differently.

Their efforts target a range of potentially problematic content, from hate speech and false information about elections to what they see as fringe views on health, along with other issues the platforms consider harmful. Pinterest recently banned posts that it deems to contain misinformation about climate change.

Twitter and other platforms also have systems allowing users to report claims of abuse by other users, which can lead to account suspensions that some of those involved say can be arbitrary or unfair.

More moderating has required complex, and often confusing, guidelines for employees and users. Such efforts have led to some divisive decisions, including bans by Twitter and Facebook of then-President Donald Trump for posts encouraging the rioters who attacked the U.S. Capitol on Jan. 6, 2021, seeking to halt certification of President Biden's electoral victory.

Those bans were widely praised on the left and by some Republicans. But Mr. Trump and his supporters blasted it and have backed rival social-media apps with fewer restrictions that so far have gained limited traction, including Parler, Gab, and Gettr, which former Trump adviser Jason Miller launched last year.

Many in the Trump camp have voiced support for Mr. Musk's planned Twitter acquisition, though Mr. Trump told Fox News on Monday that he doesn't plan to return to the platform and would instead use his social-media startup, Truth Social.

Devin Nunes, CEO of Truth Social, lauded Monday's Twitter deal, though he said he doesn't see the platform, even under Mr. Musk, as a direct competitor because of their different target audiences. "This is a great, great day," the Trump ally and former GOP congressman said. "Finally we're getting some movement in the right direction here in terms of opening the internet back up and giving people their voice back."

Twitter's existing policies haven't brought about huge financial success; it has struggled to deliver consistent profits. And losing advertisers might not be a problem for Mr. Musk. He has said he wants the platform to rely less on advertising -- which accounts for about 90% of its revenue, totaling $5.1 billion last year -- and to focus more on a subscription business model. The platform currently offers Twitter Blue, a $2.99-a-month subscription service that gives customers access to premium features.

Despite Mr. Trump's comments Monday, many observers and Twitter employees are focused on Mr. Musk's handling of the former president's account as a potential indicator of his plans. During an all-hands meeting on Monday, Twitter CEO Parag Agarwal was asked if the company might reinstate Mr. Trump once Mr. Musk's ownership is completed. Mr. Agarwal said it wasn't clear.