Why is "Current proportions" option missing from Illustrator CC 2019?

As Monika and Doug have confirmed you, that info was dropped off some versions ago.

cdMn has informed you that value showed the width to height ratio of the artboard.

While the checkmark at left Constrain Proportions, that shows a chain between width and height when is checked on, keeps the ratio w/h, and allows to increase e.g. the width by a certain amount  (+120pt) or a percentage (*1,25) and increasing the height proportionally, keeping their ratio or proportions.

Just divide wide by height and you will get the current proportions! 

It's present in CS6, but not in CC 2018 or 2019. That screenshot on the left looks like a later version than CS6, so it was presumably dropped in an earlier CC version.

On the forums we can't tell you why it's not there (because we don't know either), but if you need it, please post to http://illustrator.uservoice.com