Tested on marlin (Pixel XL, factory image, not unlocked/rooted) and sunfish (Pixel 4a, LinageOS 17.1). Attempted both mobile data and Wi-Fi.

Logcat from marlin:

12-09 01:01:19.458 27026 27057 I flutter : https://derpibooru.org/api/v1/json/search/images?q=TS%2C+(safe)&page=1&key=&per_page=30
12-09 01:01:19.461 27026 27057 E flutter : [ERROR:flutter/lib/ui/ui_dart_state.cc(177)] Unhandled Exception: SocketException: Failed host lookup: 'derpibooru.org' (OS Error: No address associated with hostname, errno = 7)
12-09 01:01:19.461 27026 27057 E flutter : 

Logcat from sunfish:

12-09 00:46:35.445  8376  8376 W DartWorker: type=1400 audit(0.0:103): avc: denied { lock } for path="/system/etc/hosts" dev="dm-0" ino=619 scontext=u:r:untrusted_app:s0:c171,c256,c512,c768 tcontext=u:object_r:system_file:s0 tclass=file permissive=0 app=com.angius.chryssibooru
12-09 00:46:35.471  8376  8376 W DartWorker: type=1400 audit(0.0:104): avc: denied { lock } for path="/system/etc/hosts" dev="dm-0" ino=619 scontext=u:r:untrusted_app:s0:c171,c256,c512,c768 tcontext=u:object_r:system_file:s0 tclass=file permissive=0 app=com.angius.chryssibooru
12-09 00:46:35.498  8376 14822 E flutter : [ERROR:flutter/lib/ui/ui_dart_state.cc(177)] Unhandled Exception: SocketException: Failed host lookup: 'derpibooru.org' (OS Error: No address associated with hostname, errno = 7)
12-09 00:46:35.498  8376 14822 E flutter : 

A lot of them throw some API deprecation warnings. Not anything breaking, but consider updating them.

warning: [options] source value 7 is obsolete and will be removed in a future release
warning: [options] target value 7 is obsolete and will be removed in a future release
warning: [options] To suppress warnings about obsolete options, use -Xlint:-options.
Note: D:\Tools\flutter\.pub-cache\hosted\pub.dartlang.org\package_info-0.4.0+13\android\src\main\java\io\flutter\plugins\packageinfo\PackageInfoPlugin.java uses or overrides a deprecated API.
Note: Recompile with -Xlint:deprecation for details.
3 warnings
warning: [options] source value 7 is obsolete and will be removed in a future release
warning: [options] target value 7 is obsolete and will be removed in a future release
warning: [options] To suppress warnings about obsolete options, use -Xlint:-options.
3 warnings
warning: [options] source value 7 is obsolete and will be removed in a future release
warning: [options] target value 7 is obsolete and will be removed in a future release
warning: [options] To suppress warnings about obsolete options, use -Xlint:-options.
3 warnings
warning: [options] source value 7 is obsolete and will be removed in a future release
warning: [options] target value 7 is obsolete and will be removed in a future release
warning: [options] To suppress warnings about obsolete options, use -Xlint:-options.
Note: D:\Tools\flutter\.pub-cache\hosted\pub.dartlang.org\path_provider-1.5.1\android\src\main\java\io\flutter\plugins\pathprovider\PathProviderPlugin.java uses or overrides a deprecated API.
Note: Recompile with -Xlint:deprecation for details.
3 warnings
warning: [options] source value 7 is obsolete and will be removed in a future release
warning: [options] target value 7 is obsolete and will be removed in a future release
warning: [options] To suppress warnings about obsolete options, use -Xlint:-options.
Note: D:\Tools\flutter\.pub-cache\hosted\pub.dartlang.org\share-0.6.3+5\android\src\main\java\io\flutter\plugins\share\SharePlugin.java uses or overrides a deprecated API.
Note: Recompile with -Xlint:deprecation for details.
3 warnings
warning: [options] source value 7 is obsolete and will be removed in a future release
warning: [options] target value 7 is obsolete and will be removed in a future release
warning: [options] To suppress warnings about obsolete options, use -Xlint:-options.
3 warnings
warning: [options] source value 7 is obsolete and will be removed in a future release
warning: [options] target value 7 is obsolete and will be removed in a future release
warning: [options] To suppress warnings about obsolete options, use -Xlint:-options.
3 warnings
warning: [options] source value 7 is obsolete and will be removed in a future release
warning: [options] target value 7 is obsolete and will be removed in a future release
warning: [options] To suppress warnings about obsolete options, use -Xlint:-options.
D:\Tools\flutter\.pub-cache\hosted\pub.dartlang.org\shared_preferences-0.5.6\android\src\main\java\io\flutter\plugins\sharedpreferences\SharedPreferencesPlugin.java:25: warning: [deprecation] getFlutterEngine() in FlutterPluginBinding has been deprecated
    setupChannel(binding.getFlutterEngine().getDartExecutor(), binding.getApplicationContext());
                        ^
4 warnings
warning: [options] source value 7 is obsolete and will be removed in a future release
warning: [options] target value 7 is obsolete and will be removed in a future release
warning: [options] To suppress warnings about obsolete options, use -Xlint:-options.
3 warnings
warning: [options] source value 7 is obsolete and will be removed in a future release
warning: [options] target value 7 is obsolete and will be removed in a future release
warning: [options] To suppress warnings about obsolete options, use -Xlint:-options.
3 warnings
warning: [options] source value 7 is obsolete and will be removed in a future release
warning: [options] target value 7 is obsolete and will be removed in a future release
warning: [options] To suppress warnings about obsolete options, use -Xlint:-options.
3 warnings
warning: [options] source value 7 is obsolete and will be removed in a future release
warning: [options] target value 7 is obsolete and will be removed in a future release
warning: [options] To suppress warnings about obsolete options, use -Xlint:-options.
Note: Some input files use or override a deprecated API.
Note: Recompile with -Xlint:deprecation for details.
3 warnings
Note: D:\Tools\flutter\.pub-cache\hosted\pub.dartlang.org\video_player-0.10.5\android\src\main\java\io\flutter\plugins\videoplayer\VideoPlayerPlugin.java uses or overrides a deprecated API.
Note: Recompile with -Xlint:deprecation for details.

• Fixed the API key not being saved
• Replaced CachedNetworkImage with AdvancedNetworkImage
• Added image cache cleanup
• Added progress bar for image loading
• Share button works
• Copy button works
• Open in browser button works
• Added app icon
• Swapped app logo from .png to .svg
• Added a Ko-Fi button
• Configured the app signing

I couldn't get any art to load no matter what I typed in however once I tried the pre-release and it seemed fixed. I will continue to play around with it to see if it is fine.

Take a look at https://github.com/Sibusten/derpibooru-downloader and how they can scrape a lot from the servers. The issue here is that the Downloader only scrapes, but does not allow you to natively search tag sets within the download.

Hi, your app seems nice, what about to publish it in F-droid open-source android "market"(more like repository)? Here FAQ - https://f-droid.org/docs/FAQ_-App_Developers/?title=FAQ-_App_Developers