I have never used this service, don't think I have heard of it before now. From what I can see they are operating in a similar manner that Luno was a couple of years back, that's another service I do not know if it's still active today.
But I am kind of surprised when I saw the email message because I do not think there would be any Nigeria that will use Yellow Card on browser instead of the mobile app.
In the email, they say the web app will not longer be available, that can be a little confusing as it's not the same thing as the web browser version. Some apps have pc version and the browser version which is different.