It turned out that Twitter prohibited the embedding of tweets on a specific site

On March 30, 2023, Twitter announced a new API plan , which has had a major impact on the operation of third-party internet services linked to Twitter. Meanwhile, it was reported that tweet embedding was restricted on Substack, a newsletter platform that can deliver content in a subscription format.

Twitter cuts off Substack embeds and starts suspending bots - The Verge
https://www.theverge.com/2023/4/6/23673043/twitter-substack-embeds-bots-tools-api

Substack is creating a Twitter-style feature called Notes
https://www.siliconrepublic.com/business/substack-notes-twitter-news-feed

Twitter has an official function to embed tweets, and it is possible to directly display a specific tweet without taking a screenshot.

However, on April 6, 2023, Substack suddenly stopped embedding tweets. Substack's official Twitter account said, 'We are investigating reports that Twitter embedding and authentication are no longer working in Substack. We are actively working to resolve this issue and will share more information as it becomes available. ' said.

We're investigating reports that Twitter embeds and authentication no longer work on Substack. We are actively trying to resolve this and will share updates as additional information becomes available.

— Substack (@SubstackInc) April 6, 2023

On April 5th, a day before tweet embedding became impossible, Substack announced a new feature called 'Notes.' With this Notes function, you can post text, images, and links and display them in a feed that is often found on SNS. It was very similar to Twitter, as he said.

A Substack spokesperson did not comment on whether the ban on embedding tweets was caused by Twitter's free API restrictions, but IT news site The Verge said, ``We are affected by Twitter's new API policy. Substack is not the only one, 'he hinted at the possibility of retaliation for Substack's Notes announcement.

However, the impact of Twitter's API policy change is also being seen on other platforms, although it won't ban embedding tweets.

Twitter not only blocks access to free APIs, but also disables logging in to external services using Twitter accounts - GIGAZINE

by Esther Vargas

For example, WordPress, an open source website building platform, has revealed that it can no longer automatically share posts to Twitter due to restricted access to the Twitter API.

Our access to the Twitter API is currently blocked. As a result, Jetpack Social is temporarily unable to automatically share your posts directly from https://t.co/eRvNKWaolr to Twitter. We have reached out to Twitter for more information on how to get unblocked.

—WordPress.com (@wordpressdotcom) April 4, 2023

In addition, Social Bearing, which provides services related to Twitter analytics, reports that access to Twitter's free API has been revoked, making it impossible to operate.

I am sad to announce that as of today, Social Bearing is no longer operational as our access to Twitter's free API has been revoked

The new free and basic API plans are far too limiting, and at ~$40k/month, the enterprise tier is far too expensive to keep running pic.twitter.com/wpGTTC8Lkp

— Social Bearing (@socialbearing) April 4, 2023