maybe it's more appropriate to say: "DiDi is losing its users" , maybe it has been replaced by other APP with more features, or there is regulation from the government that restraint people from using it.

"no longer X" can be translated as "不再X了"

I would translate it as "it's passe to...." in your given context. "No longer popular" is works in other context, such as "the song is no longer popular", "no longer in favor". How a word or phrase used in a language is culturally biased. 流行 is used in all such context. In English, "love" is used in all context but Chinese has various shades of "love", e.g. 愛, 情, 恋, 惜, 憐, 恩 and hence many other terms used to describe/distinguish the situation, just as Eskimos has around 50 shades of meaning for "snow" and in Scottish, 421!

You can also say 现在 几乎 已经 没人 用 滴滴打车 了。

Thank you for that very helpful answer!

Hi, "no longer X" basically will be translated into "不再X了" or “已经不X了”。Another version for "no longer popular" could be "过时了"、“过气了”、“落伍了”、“跟不上时代了” etc. However, these phrases are all related to "out of date"; "not IN anymore". They often refer to something from a certain time period. (For example, what is popular in the 1980s will be out of date for today.)