Update CHANGELOG.md

Author: FBumann

CHANGELOG.md

@@ -51,7 +51,7 @@ If upgrading from v2.x, see the [v3.0.0 release notes](https://github.com/flixOp
 
 ## [Unreleased] - ????-??-??
 
-**Summary**: Renamed OnOff terminology to Status terminology for better alignment with PyPSA and unit commitment standards.
+**Summary**: Renamed OnOff terminology to Status terminology for better alignment with PyPSA and unit commitment standards. **All deprecated items from v4.x have been removed.**
 
 ### ✨ Added
 
@@ -127,6 +127,44 @@ A partial backwards compatibility wrapper would be misleading, so we opted for a
 
 ### 🔥 Removed
 
+**Modules removed:**
+- `calculation.py` module - Use `optimization.py` instead
+
+**Classes removed:**
+- `Calculation`, `FullCalculation` → Use `Optimization`
+- `AggregatedCalculation` → Use `ClusteredOptimization`
+- `SegmentedCalculation` → Use `SegmentedOptimization`
+- `Aggregation` → Use `Clustering`
+- `AggregationParameters` → Use `ClusteringParameters`
+- `AggregationModel` → Use `ClusteringModel`
+- `CalculationResults` → Use `Results`
+- `SegmentedCalculationResults` → Use `SegmentedResults`
+
+**Functions removed:**
+- `change_logging_level()` → Use `CONFIG.Logging.enable_console()`
+
+**Methods removed:**
+- `Optimization._perform_aggregation()` → Use `_perform_clustering()`
+- `Optimization.calculate_aggregation_weights()` → Use `calculate_clustering_weights()`
+
+**Parameters removed:**
+- `Optimization.active_timesteps` → Use `flow_system.sel(time=...)` or `flow_system.isel(time=...)`
+- `TimeSeriesData.from_dataarray()`: `aggregation_group` → Use `clustering_group`
+- `TimeSeriesData.from_dataarray()`: `aggregation_weight` → Use `clustering_weight`
+- `FlowSystem.weights` → Use `scenario_weights`
+- `Results.__init__()`: `flow_system` → Use `flow_system_data`
+- `Results` plotting methods: `indexer` → Use `select`
+- `Results.plot_heatmap()`: `heatmap_timeframes`, `heatmap_timesteps_per_frame` → Use `reshape_time`
+- `Results.plot_heatmap()`: `color_map` → Use `colors`
+
+**Properties removed:**
+- `FlowSystem.all_elements` → Use dict-like interface (`flow_system['label']`, `.keys()`, `.values()`, `.items()`)
+- `FlowSystem.weights` → Use `scenario_weights`
+
+**Features removed:**
+- Passing `Bus` objects directly to `Flow` → Pass bus label string instead and add Bus to FlowSystem
+- Using `Effect` objects in `EffectValues` → Use effect label strings instead
+
 **Deprecated parameters removed** (all were deprecated in v4.0.0 or earlier):
 
 **TimeSeriesData:**