Hi eLearning and welcome to the Mattermost forums!
Unfortunately, nothing has changed on that topic so far, this feature is still not available. I also was used to such one-time chat links back when we were using HipChat and it was a nice way to create an easy time-based chat to work with someone external to your company on a migration or during an incident, f.ex.
The last time I asked about this feature the answer was that something like this is not planned currently.
I don’t think that it’s complex to create something like that on top of Mattermost which could, f.ex., also be used as a Live Chat plugin for supporting your customers on the website. A bot account is all it would need for that to work and some wrappers left and right.