I think you want to install version 3 and see for yourself.

Is this even possible or will it instantly upgrade itself without asking?

It's totally possible to install old versions of Chromium without them auto-updating. It's how you bisect to find when something broke

I suspect it doesn't go back to version 3 though

https://www.chromium.org/developers/bisect-builds-py

I wish I could do that with web apps too... AKA Google Search... year-2005 would be nice

With data from pre 2005 as well? If not maybe a simple greasemonkey style script could be enough.

The search algo is what I would like to see... Not how it used to look.