docs: Provide policy on breaking changes & deprecations in KEDA

DEPRECATIONS.md

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+# Deprecations & Breaking Changes in KEDA
+
+This document describes how KEDA handles deprecations and breaking changes and should provide clarity to KEDA end-users on what to expect when upgrading to a new version of KEDA.
+
+If you are interested in the current open deprecations, we recommend checking our [GitHub Discussions](https://github.com/kedacore/keda/discussions/categories/deprecations) or the open [breaking changes](https://github.com/kedacore/keda/issues?q=is%3Aopen+label%3Abreaking-change+sort%3Aupdated-desc).
+
+## Our versioning strategy
+
+KEDA follows the deprecation strategy of Kubernetes and is allowed to make breaking changes in minor versions after a deprecation period.
+
+The KEDA runtime, which consists of the operator and metric server, will provide support on the deprecated features for 4 releases/1 year after the deprecation was announced. In the following release, the feature will be removed.
+
+Depending on the area of the change, KEDA might not wait 4 releases to introduce a breaking change because one of its dependencies such as Kubernetes or scaler dependencies. However, these are the only exceptions to the rule.
+
+### Custom Resource Definitions (CRDs)
+
+Our custom resource definitions (CRDs) have a separate versioning scheme than the KEDA runtime given they have an `apiVersion`. For this, we follow the [official Kubernetes API versioning policy](https://kubernetes.io/docs/reference/using-api/#api-versioning).
+
+KEDA is allowed to make breaking changes to the CRDs, when a new `apiVersion` is introduced.
+
+### Autoscaling triggers and their metadata
+
+Until KEDA introduces a [versioning scheme for autoscaling triggers](https://github.com/kedacore/keda/issues/613), it is not allowed to make breaking changes for triggers.
+However, it is allowed to introduce new triggers and features in the same `apiVersion`.
+
+The only exceptions to this rule is when KEDA maintainers decide to remove a scaler as part of our [scaler governance](SCALERS.md). In this case, the scaler will be removed after the above mentioned deprecation period of 2 releases/6 months. The reason for being able to remove scalers is that they are no longer maintained and thus no longer supported.
+
+### Helm chart
+
+The Helm charts that KEDA provides has a separate release & version lifecycle. They follow [Semantic Versioning](https://semver.org/) but specifically around the Helm charts and is versioned independently from the KEDA runtime.
+
+### Kubernetes compatibility
+
+As per our [Kubernetes compatibility](https://keda.sh/docs/latest/operate/cluster/#kubernetes-compatibility) policy, we provide support for Kubernetes N-2.
+
+This means that a minor version upgrade of KEDA is backwards-compatible with Kubernetes N-2, but might remove support for an older version.
+
+This kind of change will be announced in the release notes and enforced in our Helm charts.
+
+You can find an overview of our compatibility matrix in our [Kubernetes compatibility](https://keda.sh/docs/latest/operate/cluster/#kubernetes) documentation.
+
+The only exception to this rule is when we are forced to make a breaking change that is enforced by Kubernetes and we need to introduce it sooner.
+
+## What is considered a breaking change?
+
+A breaking change is a change that is not backwards-compatible and requires end-users to take manual action to migrate or lose existing functionality.
+
+## Avoiding breaking changes
+
+When contributors want to introduce a new approach, they are allowed to do so without breaking changes. This means that the new approach will live next to the deprecated functionality during the deprecation period.
+
+Every occasion of this must provide warning(s) in the KEDA logs to make end-users aware of the change so they can decide whether to migrate to the new approach - Either by using the new functionality in the next minor version or by migrating to the new approach when the next major version was released.
+
+Every deprecation must follow our process described below.
+
+## Introducing new deprecations
+
+When introducing a new deprecation, it must comply with the following rules:
+
+- Every deprecation must be announced in the release notes and will be highlighted in the upcoming release(s)
+- Every deprecation must have a warning in the KEDA logs to create awareness
+- Every deprecation must have a representing issue that is used for tracking breaking changes for our upcoming major version.
+  - Because of that, it must be labeled with [`breaking-change`](https://github.com/kedacore/keda/issues?q=is%3Aopen+label%3Abreaking-change+sort%3Aupdated-desc)
+  - A label will be added to indicate the version in which it will be removed, example `removal:keda-v2.12`.
+- Every deprecation must be announced on [GitHub Discussions](https://github.com/kedacore/keda/discussions/categories/deprecations) ([example](https://github.com/kedacore/keda/discussions/3552))
+  - It should explain when the deprecation takes effect, what the impact is, how to migrate and when it will be removed.