As reported, the changes will begin to apply today, May 10. Elon Musk wrote about it on Twitter.

what you can expect

• how detailed explained in the tweet from the company’s support service, you can now responding to specific messages in personal correspondence by quoting. To do this, you need to click on the message of the addressee and hold your finger until the menu appears. You can comment on a specific message by selecting “Reply”. This is especially useful if you receive several different messages with different questions.
• Also, new reactions emerged. Select “Add a reaction” from the menu or the reaction icon at the bottom of the message, then press the three dots. You can now add any smiley from the full emoji set.

The biggest change that comes with the current update is, of course, the end-to-end encryption that Musk promised before finalizing the deal to buy the company. He described this as a priority for Twitter at a staff meeting in November. There’s no specific manifestation of this functionality as they’re hidden at the code level, but Musk said the encryption release should also happen on May 10 and will “grow in complexity” quickly.

End-to-end encryption, as the name suggests, encrypts your messages only with private keys stored on the recipients’ devices. This means that the company itself does not have access to them.

Acid test, even with a gun to my head, I won’t be able to see their DMs.
Musk wrote in his tweet on Tuesday.