If you get an error message like this: error: package android.support.annotation does not exist, Most likely your plugin doesn't support androidx yet, please do the following:

cordova-plugin-admob-free

Edit File:

1. plugins/cordova-plugin-admob-free/src/android/AdMob.java
2. platforms/android/app/src/main/java/name/ratson/cordova/admob/AdMob.java

This code:

import android.support.annotation.NonNull;

change to:

import androidx.annotation.NonNull;

Then install this plugin:

ionic cordova plugin add cordova-plugin-androidx@latest

ref: stackoverflow

cordova-plugin-ionic-webview

Edit file:

1. plugins/cordova-plugin-ionic-webview/src/android/com/ionicframework/cordova/webview/IonicWebViewEngine.java
2. platforms/android/app/src/main/java/com/ionicframework/cordova/webview/IonicWebViewEngine.java

This code:

import android.support.annotation.RequiresApi;

change to:

import androidx.annotation.RequiresApi;

Then install this plugin:

ionic cordova plugin add cordova-plugin-androidx@latest