It is starting to become clear in Marvel Comics' Star Wars series that Luke Skywalker would have discovered time travel if not for Star Wars Rebels. Luke Skywalker made the mistake of leaving Master Yoda on Dagobah long before he was ready, heading out to Bespin to rescue his friends in The Empire Strikes Back. The encounter with Darth Vader left Luke shaken to the core, horrified by the revelation he was actually the son of the Dark Lord of the Sith.
Luke didn't return to Dagobah straight away - but that doesn't mean he abandoned his quest to become a Jedi. Rather, Charles Soule's Star Wars comic has revealed Luke began to travel the galaxy, seeking lost secrets of the Jedi. In fact, he even discovered a Holocron left by Master Yoda himself - one that apparently dated back to the High Republic Era, at least 200 years before the events of Star Wars: Episode I - The Phantom Menace. Surprisingly, his quest almost led him to discover one of the greatest secrets of the Jedi Order - when he visited the Jedi Temple on Lothal in Star Wars #19, by Charles Soule and Marco Castiello.
Lothal is a planet rich in the Force, and the Jedi Temple there was constructed on a vergence in the Force from which it is possible to access a realm called the World Between the Worlds. This is a place where the Cosmic Force that binds the universe together and the Living Force which is associated with life come together in unison. From the World Between the Worlds, it is possible for a Force-sensitive to see - and even interact with - every moment in the past, present, and future. Luke would actually have discovered time travel - if not for the fact the Temple was sealed in Star Wars Rebels, with Ezra and Ahsoka preventing the Emperor gaining access to the World Between the Worlds. It's fascinating to imagine what lessons Luke could have learned from the World Between the Worlds - although, to be fair, he may simply not have been ready for the experience.
Curiously enough, evidence is building that Luke Skywalker may discover time travel in the end anyway - perhaps not even through the World Between the Worlds. Previews for Star Wars #20 have shown him interacting with a Jedi from the High Republic Era named Elzar Mann, presumably as part of some sort of mystical experience, and that seems to be tying Luke in to the transmedia initiative Star Wars: The High Republic. It's possible Luke's visit to Lothal in Star Wars #19 is a hint of the path he's on.
Whether that is the case or not, though, Luke would go on to learn more about the World Between the Worlds. The sacred Jedi texts he collected throughout his time as a Jedi Master included books that taught about the World Between the Worlds; Lucasfilm has printed some pages, and they clearly showed more about Lothal. Luke Skywalker may not have possessed the key to unlock the Jedi Temple on Lothal in Star Wars #19, but he may well have gained it in the end.