Hi, think of it in two parts: rules and actions. They both come in types and the subscription depends on the type. Fortunately it is quite straightforward:
The majority of rule types are on the free subscription. So for example you can create, manage and use "generic" index threshold rules, ES query rules and Observability rules on the UI or using the public API and you can leverage the full functionality on the free subscription. No limitations to this part, you enjoy the exact same capabilities with paid subscriptions. However, two rule types are not on the free subscription: Geofencing (Maps) and anomaly detection (ML) rules. These are marked on our subscriptions page as such.
Regarding actions: Connectors that send events outside the Elastic stack (webhooks, email, Slack, PagerDuty, ServiceNow, Jira etc.), are on Gold+ subscriptions. In contrast, indexing your alerts in Elasticsearch or showing them as server logs are on the free tier.
So in short and depending on your specifics, if you do not need an integration with a third party system to send your alerts across and if you do not need geofencing or ML anomaly detection alerts, then you can use Kibana alerting in full on the free subscription.