Warning: Contains Spoilers for Stargirl season 2, episode 1.
Jay Garrick’s The Flash once resided.
Despite being a branch of the Arrowverse’s rebooted multiverse, Stargirl's existence on Earth-2 remains unknown to the superheroes of Earth-Prime. In Stargirl’s season 2 premiere, however, Pat Dugan revealed that one of the Justice Society of America’s villains, Per Degaton, who was one of the of the Injustice Society of America, was defeated and banished to an alternate timeline by The Flash. There aren’t any further details given surrounding what this alternate timeline entails or how it was created, but the information does allude to the existence of time travel and, crucially, parallel worlds.
Considering Crisis on Infinite Earths created a streamlined universe, alternate timelines have not really existed since. In this instance, an alternate timeline likely means Stargirl’s The Flash sent Per Degaton to another earth, essentially confirming the speedster is at least somewhat aware of the multiverse (or at least that another Earth exists). As established by Flashpoint in season 3 of The Flash and in Legends of Tomorrow, time traveling can create alternate timelines by erasing the one that originally existed. However, both can’t exist at the same time and one always ends up wiping out the other if it cements itself after a while. For The Flash to banish Per Degaton to an alternate reality means he’s alive somewhere in the multiverse and is no longer on Earth-2.
Interestingly, Per Degaton is someone who is out of sync with normal time and is aware when changes happen to the timeline. In the comics, he is able to the timeline that existed before Crisis on Infinite Earths as well as travel through time, so Stargirl specifically naming this villain in the season 2 premiere seemingly foreshadows what’s to come with the arrival of John Wesley Shipp’s Jay Garrick, whose presence on the show will reopen the doors to the multiverse. What’s more, the multiverse connection reaffirms The Flash as the link to alternate timelines and multiversal travel. As a speedster, The Flash’s ability to vibrate allows him to travel between worlds and dimensions, which was how Barry Allen was able to crossover to Earth-2 and Earth-3 on The Flash.
It’s possible Stargirl season 2 will reveal Garrick’s iteration of The Flash crossed over to another world. It could explain why everyone presumes he’s been dead for all these years and how he might return in the present day. Whatever the case, Stargirl is taking the first steps to fully establish itself as a part of the larger Arrowverse with the mention of Per Degaton and alternate timelines. This sets up the likelihood of the multiverse worlds interacting with each other once more instead of remaining separate, and could lead to Stargirl and the new of the JSA finally interacting with the other superheroes on Earth-Prime.