Shadowdark (which is where I'm leaning at present, although my gaming group is currently on hiatus) stops at 10th, and I'd call it "Old-school D&D compatible." Kelsey took the streamlined design of modern games (Core check mechanic: "roll d20, high is good") and reworked everything around it with the deliberate intent to create a game with a solid old-school "feel."
I saw this post in the middle of the night, but I have more time this afternoon to respond.
While I agree that Shadowdark evidences modern design, I don't necessarily agree with your characterization here that "roll d20, high is good" is necessarily somehow more modern to the seeming exclusion of other design choices a game may make for its resolution system. I agree that it's more intuitive and (subjectively) better than THAC0 and more old school D&D games/clones that adhere to it.
Let's again take Dragonbane. It's roll under. However, its d20 roll under skill-rating system is indebted to the IP's origins in BRP/RuneQuest, which is also why it has ducks. In effect, this means that the GM is not establishing a DC beforehand, nor are the players are not looking at the GM to see if they succeed. IME, this roll under system tends to be much quicker in play than d20 vs. AC as players know
as soon as they roll whether they succeed or fail. If you don't think that this streamlines and quickens things at the table, then I don't know what else to tell you.
I would also say that Dragonbane evidences more modern design elements than Shadowdark. After all, the term "Neo-Trad" was coined in 2015 by Tomas Härenstam to describe the sort of games that Free League were creating: “
it’s got the production values, ease of use and plentiful campaign material of a traditional RPG, combined with the kind of clever and thematic rules design usually found in the indie games.” This originally applied to their Year Zero Engine system, but IMHO it definitely carries over into Dragonbane.
There's a bunch of other things I like about it, like Kelsey's approach to monster design (I'd call it "5e-Lite:" most monsters get A signature ability that lets them stand out from the rest, but they don't get a bunch of them).
When a GM attacks with a Monster, the GM rolls on the Monster's unique attack chart. IME, this also makes thing quick, interesting, and surprising for the GM, reducing the "what should I do?" mental workload for the GM with monster actions.
Check DCs range from 9 to 18.
There are no GM-designated DCs in Dragonbane as players are rolling under their PC's derived skill level. However, circumstances may confer Boons and Banes on those rolls.
And even a 10th-level fighter tops out at an average of 50 hit points.
By default a PC's total number of HP in Dragonbane equals their Con score. That's it. There are no levels in Dragonbane so HP remains consistent, though there is one heroic ability that can increase HP (i.e., Robust). So your Fighter's max HP is likely 18 HP at the beginning and end of the campaign.
Two of the big selling points for me were that one, I really like Kelsey's reworking of magic; not just that she made a non-Vancian system, but also that she totally condensed and rewrote the D&D spell list. I'm sure there are some spells that are "missing," but I couldn't offhand tell you what they are (with the exception of Raise Dead and its more potent brethren) in only 5 spell "Tiers."
Dragonbane uses a hybrid-Vancian magic system with both roll to cast and mana points.
A mage chooses their school (Animism, Elementalism, or Mentalism). There is no Wizard/Arcane vs. Cleric/Divine divide. Mages have a skill level in their school. A mage rolls against their skill level to succeed. They memorize and prepare spells equal to their base Int level. Spells also cost Willpower points, though if a Mage has no Willpower points, they can also fuel their spells with HP. Mages can also cast spells in their grimoire that they have unprepared but these take double the time. There are also possible bonus effects or mishaps when rolling a Dragon (Nat 1) or Demon (Nat 20).
And secondly, I really like that it still uses Advantage/Disadvantage, Death Saves and has a version of Inspiration (Luck Tokens). Those are all 5e innovations that I love. (Another 5e thing I adored was proficiency dice, but I think I was one of the only ones).
Advantage/Disadvantage is part of Dragonbane in the form of Boons and Banes. Banes and Boons cancel out. However, in contrast to 5e Adv/Dis, Boons and Banes can stack. So for example, you can have two Boons (roll 3d20, take the lowest) or two Banes (roll 3d20, take the highest). Players can optionally push their rolls to try again by voluntarily taking on a Condition after their roll, which confers a Bane on rolls associated with that attribute.
Death Saves are also part of Dragonbane. Three successful rolls against your Con results in recovering d6 HP. Three failed rolls against your Con results in Death. Additionally another PC within 10 meters can Persuade you to Rally, so you can keep fighting at 0 HP while also making death saves.
There is no Inspiration; however, the option for PC re-rolls are handled through the Push Your Roll mechanic, which I mentioned above. I tend to like this much better than Inspiration as this puts this in the players' hands and not fishing the GM for Inspiration with "good roleplay" while also having a cost (Bane on skills associated with attribute) and limit (one condition per attribute) associated with it.
However, it's also super-simple (if not always "easy") to create classes for.
Professions in Dragonbane amount to a heroic ability, skills, and a d6 roll for starting equipment.