Font Height Options
This feature, first introduced in Illustrator 2020 (version 24.3) in August of 2020, allows users to set font size not only traditionally, by its line height, which includes ascenders and descenders, but also based on lowercase and capital letters’ height.
To access this function, we should open the Character panel’s flyout menu and enable 'Font Height Options' option. This adds one more dropdown in the panel and modifies the look of the Font Size field’s icon, reflecting the chosen font height reference.
You can read the official help article here: https://helpx.adobe.com/illustrator/using/align-text.html
If you have any feedback specific to this feature, you can leave it here in comments and log a separate report / request here at UserVoice, or upvote existing entries.
Some ideas and bug reports on the topic were already reported:
Font Height Options Tab order is broken
Font Height Options misbehave with rounded fonts
Cap Height setting not using Font Cap Height