Some big tech organizations basically do this without the pay. Essentially in some FAANG corps, you have to prove having worked at L+1 while still at L to get promoted to L+1.

In my biased / personal experience, the veracity of such a bar kinda deteriorates over time and the short average tenure makes most promo processes corrupt B.S. when folks are more incentivized to just quit and join somewhere else for more money.

Also, this process and your suggestion at a "probation" tends to motivate working harder for the window being measured, regardless of how actually adept or well equipped the person is at the level for which they're trying to illustrate competence.