Version 28 is the last version of the legacy support library, so we recomand that you migrate to AndroidX libraies

According documentation to use Notifications I past this line of code
implementation "com.android.support:support-compat:28.0.0" to build.gradle file. But Android Studio underlines it by red and says: Version 28(intended for Android Pie and below) is the last version of the legacy support library, so we recomand that you migrate to AndroidX libraies when using Android Q and moving forward. Can somebody tell me please what it means? I work with it only few days. This test project should be compatible with Android 5.0 so what is the problem with this documented back compatibility library? Here is the code from builde.gradle file:

apply plugin: 'com.android.application'

android {
    compileSdkVersion 29
    buildToolsVersion "29.0.3"

    defaultConfig {
        applicationId "web.php5.anotifikacie"
        minSdkVersion 21
        targetSdkVersion 29
        versionCode 1
        versionName "1.0"

        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }

}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])

    implementation 'androidx.appcompat:appcompat:1.1.0'
    implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'androidx.test.ext:junit:1.1.1'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'
    implementation "com.android.support:support-compat:28.0.0"
}

> Version 28(intended for Android Pie and below) is the last version of
> the legacy support library, so we recomand that you migrate to
> AndroidX libraies when using Android Q and moving forward.

AndroidX replaces the original support library APIs with packages in the androidx namespace. Only the package and Maven artifact names changed; class, method, and field names did not change.

You should use

implementation 'androidx.legacy:legacy-support-v4:1.0.0'

You should follow Release Notes