It's something I've advocated for in various forms for a while. It's an effective way to serve DM and designer interests. Half of the problem with the high levels is not having a solid design bible that indicates what kind of challenges and narratives fit into which level brackets, and the same problem from the DM side is not knowing that you can't run a high level pirate campaign because ship travel is pointless in that range. Ideally, you both want to avoid
I would steal the classic BECMI formula and mash it up with something like E6, and then categorize not only adventures published by level range, but also supplements. You would design classes that finish inside a tier, and higher tiers would list the appropriate classes for entry, so you're going Wizard->High Mage->Archmage and so on. Some archetype reshuffling would also be helpful. I'd probably do something like Fighter->Paladin, and Fighter->Swordmaster both as expert options for example. Then I'd make sure to publish systems and tools with a clear reference to the tier they work inside of. Ship combat rules are maybe an Expert module, domain management might also fall inside there.
Individual campaigns will then target a specific tier, based on what they're trying to do. You'd probably generally start at the beginning of a given tier to allow for clear progression, and some subsystems could allow for extension beyond levels, like domain management, or you could do something like E6 slow drip feat acquisition. DM supplements can provide not just additional monsters, but appropriate challenges, discussions of how distance and time affect players inside the tier, when site-based adventures make sense, how encounter design must shift when players can be expected to set the terms of engagement consistently, and so on.