Show deprecation notice for API 21.

app/src/main/java/org/thoughtcrime/securesms/banner/banners/DeprecatedSdkBanner.kt

@@ -0,0 +1,49 @@
+/*
+ * Copyright 2024 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.thoughtcrime.securesms.banner.banners
+
+import android.os.Build
+import androidx.compose.foundation.layout.PaddingValues
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.res.stringResource
+import androidx.compose.ui.unit.dp
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.flowOf
+import org.signal.core.ui.compose.DayNightPreviews
+import org.signal.core.ui.compose.Previews
+import org.thoughtcrime.securesms.R
+import org.thoughtcrime.securesms.banner.Banner
+import org.thoughtcrime.securesms.banner.ui.compose.DefaultBanner
+import org.thoughtcrime.securesms.banner.ui.compose.Importance
+
+class DeprecatedSdkBanner() : Banner<Unit>() {
+
+  override val enabled: Boolean
+    get() = Build.VERSION.SDK_INT < 23
+
+  override val dataFlow: Flow<Unit> = flowOf(Unit)
+
+  @Composable
+  override fun DisplayBanner(model: Unit, contentPadding: PaddingValues) = Banner(contentPadding)
+}
+
+@Composable
+private fun Banner(contentPadding: PaddingValues) {
+  DefaultBanner(
+    title = null,
+    body = stringResource(id = R.string.DeprecatedSdkBanner_body),
+    importance = Importance.ERROR,
+    paddingValues = contentPadding
+  )
+}
+
+@DayNightPreviews
+@Composable
+private fun BannerPreview() {
+  Previews.Preview {
+    Banner(contentPadding = PaddingValues(0.dp))
+  }
+}

app/src/main/java/org/thoughtcrime/securesms/conversationlist/ConversationListFragment.java

@@ -89,6 +89,7 @@ import org.thoughtcrime.securesms.banner.BannerManager;
 import org.thoughtcrime.securesms.banner.banners.CdsPermanentErrorBanner;
 import org.thoughtcrime.securesms.banner.banners.CdsTemporaryErrorBanner;
 import org.thoughtcrime.securesms.banner.banners.DeprecatedBuildBanner;
+import org.thoughtcrime.securesms.banner.banners.DeprecatedSdkBanner;
 import org.thoughtcrime.securesms.banner.banners.DozeBanner;
 import org.thoughtcrime.securesms.banner.banners.MediaRestoreProgressBanner;
 import org.thoughtcrime.securesms.banner.banners.OutdatedBuildBanner;
@@ -712,6 +713,7 @@ public class ConversationListFragment extends MainFragment implements Conversati
 
   private void initializeBanners() {
     List<Banner<?>> bannerRepositories = List.of(
+        new DeprecatedSdkBanner(),
         new DeprecatedBuildBanner(),
         new UnauthorizedBanner(requireContext()),
         new ServiceOutageBanner(requireContext()),

app/src/main/res/values/strings.xml

@@ -1170,6 +1170,9 @@
     <string name="ExpiredBuildReminder_this_version_of_signal_has_expired">This version of Signal has expired. Update now to send and receive messages.</string>
     <string name="ExpiredBuildReminder_update_now">Update now</string>
 
+    <!-- A message in a banner shown at the top of the screen indicating that the user must upgrade their device to use signal. -->
+    <string name="DeprecatedSdkBanner_body">Signal no longer works on this device. To use Signal again, update your device to a newer version of Android or switch to a newer device.</string>
+
     <!-- Title of notification telling users to update Signal -->
     <string name="DeprecatedNotificationJob_update_signal">Update Signal</string>
     <!-- Message body of notification telling users that this current version of Signal has expired. If pressed, we will direct them to the website APK or PlayStore page -->