Twitter has put restrictions on how its users interact with posts that contain links to the newsletter platform Substack, just days after Substack announced it would launch a Twitter-like feature called "Notes."
When trying on a desktop computer to retweet or like a post with a Substack link, Twitter returned a message: "Some actions on this Tweet have been disabled by Twitter."
On the mobile version of Twitter, likes and retweets of posts that shared a Substack link did not work.
It also appeared that Twitter had made it so that users could not respond to tweets that included Substack links.
Twitter did not immediately return a request for comment.
It appeared, however, that tweets with links to Substack newsletters with a unique domain name or shortened links did function normally on Twitter.
One user said they were also unable to embed tweets in their Substack.
The restriction appeared to have started sometime between Thursday evening and Friday morning. Some Twitter users directed their anger over the change at Twitter CEO Elon Musk.
"Defender of free speech indeed," tweeted academic and author Brian Klass.
Writer Parker Molloy suggested the move showed that Musk was willing to hurt businesses that use Twitter should he feel slighted by a person or company.
Jared Holt, a resident fellow at the Atlantic Council’s Digital Forensic Research Lab, tweeted that Musk was disincentivizing Twitter users, rather than Substack users, by limiting their ability to share newsletters from the platform.
"Feel like a broken record on this but Elon Musk fundamentally does not understand the power dynamics of a platform like this," he tweeted, adding in a subsequent tweet: "The value of this website is created by its power users. If you punish them and remove incentives for them to be here, your platform will fail. I haven’t taken a business class in my life and I can identify that much."
In a statement emailed to NBC News, Substack founders Chris Best, Hamish McKenzie and Jairaj Sethi said they were "disappointed that Twitter has chosen to restrict writers’ ability to share their work."
"Writers deserve the freedom to share links to Substack or anywhere else. This abrupt change is a reminder of why writers deserve a model that puts them in charge, that rewards great work with money, and that protects the free press and free speech," the statement read. "Their livelihoods should not be tied to platforms where they don’t own their relationship with their audience, and where the rules can change on a whim."
Substack was launched in 2017 and has grown in popularity among writers since. Twitter became a destination for those with Substack newsletters to share their work and accrue new subscribers. Although some Substacks are free, others charge subscribers a fee in order to access their newsletter.
Substack's newly announced "Notes" feature is similar to Twitter both in appearance and functionality. Users can "restack" a post, which shares that post in the same way as a "retweet," in addition to liking and commenting on it.
Security researcher Jane Manchun Wong questioned why Twitter was responding so harshly to Substack’s "Notes" feature.
"I don’t get why Twitter is having such a strong reaction over Substack’s Twitter lookalike," she tweeted. "People signed up on Mastodon a while back but are still sticking around here. The community on Substack Notes will be more niche in comparison. I wouldn’t be too worried."