Just how old an Android device in the developing world do you not want to support? Life's great at the forefront of technology, but there's a balancing act to be able to support older technology vs the bleeding edge.
I like the sentiment, but building a website that can actually function in that setting isn't a matter of mere polyfills. You need to cut out the insane bloat like React, Lottie, etc., and just write a simple website, at which point you don't really need polyfills anyway.
In other words, if you're pulling in e.g. regenerator-runtime, you're already cutting out a substantial part of the users you're describing.
A quick search tells me that firefox 143 from 6 months ago supported android 5 (Lollipop).
So that's my cutoff.
Android phones update to the latest version of Chrome for 7 years. As long as you're using browser features that are Baseline: Widely Available, you'll be using features that were working on the latest browsers in 2023; those features will work on Android 7.0 Nougat phones, released in 2016.
Android Studio has a nifty little tool that tells you what percentage of users are on what versions of Android. 99.2% of users are on Android 7 or later. I predict that next year, a similar percentage of users will be on Android 8 or later.
3.9 billion android users, means that 0.8% is 31 million people - and for a very small number of developers most of their users will be from that slice. For most of them… yeah go ahead an assume your audience is running a reasonably up to date os
Websites built with tons of polyfills are likely not run on these devices anyway, since they will run out of RAM before, let alone after they will only load after sone minutes because of CPU limitations on top of not being loaded because their x509 certs are outdated as well as the bandwith they support is not suitable to load MB sited pages