According to Catholic philosopher and apologist Michael Liccione:
The reason for that brings me to the second disanalogy: that between the partial and developing stage of divine revelation recorded in the OT and the full and definitive stage recorded in the NT. In the former state, an infallible magisterium was unnecessary because “salvation history” had not yet exhibited its focus: the God-Man Jesus himself and the “Christ event.” The purpose of an infallible magisterium is to maintain the deposit of faith whole and entire, without addition, subtraction, or corruption. It does that by adjudicating with divine authority among competing interpretations of the deposit’s sources of transmission. Yet the question how to do that could not arise prior to Jesus because the deposit had not yet been fully given, and thus was not yet whole and entire. It was still developing…It was not enough to treat the writings now included in the NT canon as the word of God; indeed, that was not even relevant. For when John wrote what I’ve cited, no such collection had even been made, and he appealed to no such authority.
I’ve already commented on this, but now I wish to address it from another angle. A popular Catholic tactic is to “disprove” sola Scriptura by quoting certain verses that mention oral modes of transmission.
Protestants counter by pointing out that sola Scriptura doesn’t apply to the era of public revelation. At the same time, it was still important to commit revelation to writing even in the age of public revelation. The Mosaic covenant is a paradigm-case.
Notice, however, that Catholics don’t apply the same standard to their own rule of faith. On the one hand, they say the necessity of sola Scriptura could only be true if that was always the case. On the other hand, they don’t say the necessity of the Magisterium could only be true if that was always the case.
They allow the magisterium to be a later development. They allow that to be a break with the past.
Indeed, savvy Catholics take that a step further. Not only do they admit there was no magisterium in OT times, but they admit that there was no magisterium in NT times. They apply Newman’s principle of development to the papacy.
On the one hand, they reject discontinuity where the Protestant rule of faith is concerned while they embrace discontinuity where the Catholic rule of faith is concerned. The magisterium can be a later development, but not sola Scriptura. The papacy can be backloaded, but sola Scriptura must be frontloaded. The magisterium can evolve, but sola Scriptura was be in place from the beginning. Darwinism for the papacy, but creationism for sola Scriptura.