Instagram is bringing back the chronological feed, sort of, as it's made some new changes to the feed that are aimed at improving the experience for the user. As to the changes, and why this is only "sort of" the return of the chronological feed is because the newest posts are not necessarily guaranteed to show up at the top of your feed and then go from newest to oldest. That said, Instagram's changes do make it so that the newest posts are "more likely" to show up towards the top then before, so chances are you will likely see the newest posts first most of the time.
Prior to these most recent changes Instagram had altered the feed so posts that it thought you wanted to see based on past interactions would show up first. These were the "recommended posts that were rolled out in December of last year." Users weren't happy with this change as it resulted in many people seeing posts that they cared less about, and after listening to feedback from its users the feed was finally changed back for the most part. Instagram is also taking away the automatic refresh function.
For those unfamiliar, Instagram was updated some time ago to allow for an automatic refresh of the feed just about any time you went back to the main page. This was another change that users found irritating as it often ended up causing posts to get lost. With this latest update, feeds will no longer update on their own. Instead, there will be a "New Posts" button at the top that users can tap if they want to refresh the page, giving them more control over when refreshes happen. All of these changed should already be live but the update has just started to roll out over the last day, so it may take a few days before these features are visible to you.