Modern Instagram includes patches for vulnerabilities. An old APK from 2019 might have known exploits that hackers can use to hijack your account.
Older versions (specifically ) still had the heart icon for activity, a chronological feed option (or a less aggressive algorithm), and no permanent "Shop" tab at the bottom.
Instagram's login servers change over time. Versions older than 2018 often cannot connect to modern authentication servers. Solution: You need an APK from at least 2019 (version 100+).
If newer is supposed to be better, why is the demand for old versions skyrocketing? Here are the top reasons users are downgrading.