Yo yo!
I have very little technical knowledge so please excuse any senseless mistakes.
I want a phone that I can use that is private and secure for the next couple of years. I understand that new android phones will be made differently so I can’t side load (whatever that means, I just know it is important). I have an iPhone 12 rn but I’d rather leave the iOS ecosystem. Rn I’m typing this message on a pixel 3.
Is a pixel 3 old enough to still be a safe and secure phone that I can side load apps on in the future?
A Pixel 3 is a very insecure phone due to the lack of security updates from Google.
The only way Google can block side-loading of apps (installing apps from F-Droid app store for example) is to use Google software. Likely this will be the Play Store or Play Services. If you install a degoogled OS, such as GrapheneOS (GOS), onto a newer Pixel you will not be blocked from side-loading.
GOS can be installed on Pixel 6, 7, 8, 9 and 10 when it’s available. Here is a page that tells you how long the various Pixel models will have security updates https://endoflife.date/pixel so you can weigh the cost v number of years of security updates.