Hello, from my personal point of view, the class divider is often used with CSS3 / HTML5. It's not useless CCS stuff.
In my view, abolition is not a step into the future. Only, it does not help the authors - either way - from my point of view, there is currently a huge step backwards instead of nourishing this line.
It is a step backwards, which also distributes tasks differently. It is too easy to make a judgment about a requirement, justifying it with an argument, it represents useless CSS instead of sensibly changing solutions.
Whether vertically or horizontally or in combination in various HTML files, it does not make sense in many cases from the perspective of the respective authors.
*) Because they used the class.
And if this one-time entry is abolished, then it should also be abolished consistently: Correct
If this is necessary from the developer's point of view, I would like to ask the question again, what led to this decision?
One line ... to abolish ...
A display: none;
doesn't prevent me from doing it, for example a "display: block;" apply and e.g. to incorporate valuable new CSS3 concepts.
And "post" is not the same thing to incorporate great CSS nations, features in that position, or otherwise. Divider made sense - either way.
I am happy to solve my own concepts. And still there is a bitter aftertaste. It's about phpBB and not basically about my concepts.

Friendly greetings
*) Edit: Especially when it comes to pure child designs etc.
And / or integrations, inheritance play an important role ...
etc. etc.