The Time to Read block was implemented in #43403 and shipped into the Gutenberg plugin in February 2023, but is not yet bundled with the core.

Issues like #13796, #15350 have been reported related to this block, but all issues labeled with this block are closed and there are no open issues. Therefore, I'm hoping this block will be included in WordPress 6.4.

However, this block relies on the new wp_word_count() function to calculate the number of characters on the server side. A patch has been submitted in the core ticket for this new function, but in order to ship this block in the core, I believe this function needs to be implemented in the core first.