I think this is my new favorite RPG.
It preserves the streamlined, intuitiive gameplay of 5E, which means no wonky, crunchy saving throw or to-hit tables (or THAC0), just straightforward ability + proficiency rolls vs DC, or (ascending) AC, for just about all checks.
Characters classes, meanwhile, are scaled back to the B/X or Rules Cyclopedia style fighter, rogue, priest, and magic-user, with options for race-as-class dwarf, elf, and halfling. All classes are simplified from 5E and there are no archetypes or subclasses to choose from. There are, however, a few meaningful options for each class which adds some nice variation and flexibility to the game. The fighter gets a fighting style, for example, while the priest and magic user each have a few options that determine which spells they can use and a few other features.
Monster and spell lists are truncated from the 5E core rules. You get just about all the classic material you could want, though I would have liked to see the inclusion of certain spells (Fire bolt cantrip) and at least one metallic (gold) dragon. Stat blocks and spell descriptions are also more concise and simpified, but they remain 5E at the core.
The game is designed to be fully compatible with 5E and I think is largely accompishes this. The default rules-as-written are designed to be slightly harsher and more "old school" than 5E. A long rest only restores 1 spent hit die, rather than full hit points, and optional rules are included to make the game even more old school. There are options for eliminating at-will cantrips, or reducing bonuses for high attributes to B/X table (where 18 only gives +3 rather than +4). At the same time, the written rules are pretty transparent with their differences from 5E, so it's easy to house rule just about anything back to 5E rules if preferred.
The DM's section on designing and running dungeons, adventures, campaigns, and so on is concise but packs a lot of good tips for getting started.
The game supports play up to level 10, so there's no "high-level content"... no age categories for dragons in the monster listings, for example, or any other insanely overpowered monsters or spells. Personally, I think this is fine. 10 levels of core D&D gameplay is plenty. More of the same at higher levels can bog down the gameplay with excessive number crunching, though 5E is much better about this than its immediate predecessors. On the other hand, back in the old Rules Cyclopedia, the game changes into something almost completely different at 9th level, aka "name level"... with rules for domain management, mass combat, and paths to immortality.
While I've always enjoyed the concept of a successful heroic adventurer eventually owning a castle, leading armies, and so on, core D&D gameplay wasn't really designed for that. Still, Into the Unknown is simple enough that you could probably fiddle around with Rules Cyclopedia content or later third-party supplements if you really wanted to include domains, warfare, etc., in a campaign at or after 9th or 10th level.
5E made a lot of improvements to the D&D "engine" that I'm personally fond of. These generally get lost or ignored by most "OSR" games, even the ones that aren't straight "retroclones" of the original cumbersome rulesets. Into the Unknown is different. It achieves its goals of playing streamlined like 5E while recapturing the simplistic design and feel of the much older editions of D&D.
All in all, an excellent RPG.