There aren't many new flagship phones which still have replaceable batteries. Most smartphone manufacturers have taken to releasing phones with non user-replaceable batteries and the current alternative is generally buying an older Android device which is dependent on manufacturer released OS updates, which can be few and far between. Manufacturers also phase out OS releases and security patches at some point for each phone which also prompts the next upgrade cycle.
https://www.androidauthority.com/bes...attery-697520/
Also
https://mashable.com/2018/01/01/why-.../#V0n.inkBxPq6

Another thing one can do is to look for a phone with a good repairability score, something common enough to find parts for 3-4 years, manufacturer with decent record of releasing security updates for older models, and find a good phone repair shop (for most, unless you're fairly handy with gadget repair).
https://www.ifixit.com/smartphone-repairability

There are certain Android related companies which are notorious for being slow with security patches & updates while others run stock. Some have a published security update lifespan or you can check the model you could have purchased X years ago for security and/or OS updates.
https://support.google.com/nexus/ans...#nexus_devices
https://www.reddit.com/r/Android/com...ate_lifespans/