Dr. M. Saalih Ali Mustafa, in his book النسخ في القرآن الكريم - مفهومه وتاريخه ودعاواه, page 55, refutes this claim on two grounds:
- How can a proper etiquette be abrogated?
- 24:59 talks about adults and 24:58 talks about children.
His first point is arguable, because if anything may be abrogated, a proper etiquette may be abrogated and replaced with an even more proper etiquette.
His second point is indeed the refutation argument. 24:59 specifies what happens when the children mentioned in 25:58 grow up. They now have to knock at all times.
Dr. Mustafa says that the impetus for abrogation is that because grown children now have to knock, that underage children don't. If that indeed is the reason why some scholars thought that abrogation took place here, then I respectfully disagree. 24:59 does not say that underage children no longer have to knock, so why would anybody think it does?
Perhaps scholars thought that servants, mentioned in 24:58, are adults, so do they have to knock in the three times only or all the time? Well, 24:59 does not speak about them specifically, so I'd say that the ruling of 24:58 stays: they don't have to knock except at those three times.