I had one of those old phones and it was preferable to have slower performance overall as opposed to seemingly random shutdowns. In reality the shutdowns were directly related to when the cpu was drawing more current than the battery could provide often on websites that had tons of client side scripting.

So before the update I’d be browsing the web and my shit would just shut off and after it would run little slower but stay on.

Iirc they ended up improving the battery management software so it would maximize battery life and suggest when it was time to have it replaced.