This feature would lock certain charts (for example, the highest difficulty chart in the beatmap) until the player has passed a reasonable rating in the previous (which should be simpler) chart. For example, the player could unlock the Expert chart (or equivalent or better) after getting an A in the Insane (or equivalent) chart.

This should not be enabled by default, so it is up to the mapper to decide.

Reasons to implement:
1. Step-by-step progression system to keep the players engaged
2. Prevents tryhards to play exceedingly difficult chart compared to their skill level (just like a 6 digits should not try 7 stars maps when they obviously can't)
3. Makes sure the ranking system works as intended because some maps are deemed "special" and is supposed to serve the core players only. If everyone just use NF that refutes the point of making special charts.
4. Prevents cheaters to abolish the ranks by locking them out preemptively as well
5. Show that your map has put in a lot of effort and is not some pp farming shit because this implicitly tells you that you need to invest time on it

Similar art: Chunithm/Phigros (to some extent, maimai as well)

In Chunithm you can unlock the Master chart after getting S rating or above in Expert chart. Of course you would still have the option to "unlock" (or in other word, unrestraint) it using "master unlock ticket" in the start of the game, but it only last for that game.

After that you still need to pass the Expert chart to truly unlock the Master chart in normal play.

Similar art: Wacca

Wacca works the same with Chunithm, except the unlock ticket is free for player to choose which map to unlock, has one time use (with an upper limit of 5) and the unlock is permanent.

Similar art: Arcaea

Most Future maps are unlockable via fragments, or if the player progressed well ahead in the story mode.

Unfortunately osu is a f2p game so there is no point in implementing ticket unlocks.

1. Step-by-step progression system to keep the players engaged

Why would the lack of a progression system not keep players engaged?

2. Prevents tryhards to play exceedingly difficult chart compared to their skill level (just like a 6 digits should not try 7 stars maps when they obviously can't)

Why is this a problem if that's what keeps them engaged? osu! is full of 6 digit players boasting how they can barely pass 6 or 7 star jumps maps and they are having fun doing it.

3. Makes sure the ranking system works as intended because some maps are deemed "special" and is supposed to serve the core players only. If everyone just use NF that refutes the point of making special charts.

You need elaborate on this. I am not sure what you mean by "special charts" and how NF mod makes them pointless.

4. Prevents cheaters to abolish the ranks by locking them out preemptively as well

So this makes me believe that you are not proposing an opt-in feature, but a permanent feature. I think that is worse. But also this complicates things 10000x more as you consider multiplayer, mod combinations, player enjoyment playing maps they can barely do, etc. Not to mention a cheater can just cheat their way to unlocking the hardest difficulties to begin with.

Also, how should it work on aspire/2b type of maps?