feat!: remove old finders flag and legacy finder logic

CHANGELOG.md

@@ -4,6 +4,10 @@ Changelog
 NOTE: isort follows the [semver](https://semver.org/) versioning standard.
 Find out more about isort's release policy [here](https://pycqa.github.io/isort/docs/major_releases/release_policy).
 
+### Unreleased
+
+  - Removed `--old-finders` and `--magic-placement` flags and `old_finders` configuration option. The legacy finder logic that relied on environment introspection has been removed (#2445) @joao-faria-dev
+
 ### 6.1.0 October 1 2025 
 
    - Add python 3.14 classifier and badge (#2409) @staticdev

docs/configuration/options.md

@@ -1213,19 +1213,6 @@ src_paths = ["src", "tests"]
 
 ```
 
-## Old Finders
-
-Use the old deprecated finder logic that relies on environment introspection magic.
-
-**Type:** Bool  
-**Default:** `False`  
-**Config default:** `false`  
-**Python & Config File Name:** old_finders  
-**CLI Flags:**
-
-- --old-finders
-- --magic-placement
-
 ## Remove Redundant Aliases
 
 Tells isort to remove redundant aliases from imports, such as `import os as os`. This defaults to `False` simply because some projects use these seemingly useless  aliases to signify intent and change behaviour.

docs/upgrade_guides/5.0.0.md

@@ -67,8 +67,7 @@ In isort 5, you can still get this behavior by moving your extra modules from th
 ### module placement changes: `known_third_party`, `known_first_party`, `default_section`, etc...
 isort has completely rewritten its logic for placing modules in 5.0.0 to ensure the same behavior across environments. You can see the details of this change [here](https://github.com/pycqa/isort/issues/1147).
 The TL;DR of which is that isort has now changed from `default_section=FIRSTPARTY` to `default_section=THIRDPARTY`. If you all already setting the default section to third party, your config is probably in good shape.
-If not, you can either use the old finding approach with `--magic-placement` in the CLI or `old_finders=True` in your config, or preferably, you are able to remove all placement options and isort will determine it correctly.
-If it doesn't, you should be able to just specify your projects modules with `known_first_party` and be done with it.
+If not, you should be able to just specify your projects modules with `known_first_party` and be done with it.
 
 ## Migrating pre-commit