Here are the supported types (from /services/data/v58.0/actions/custom)

• apex for calling @InvocableAction annotated methods
• flow for starting Flow interviews (including mass/bulk runs)
• quickAction for calling Global Publisher actions

I found them via Workbench REST Explorer and tested each in Apex. The unsupported ones throw an error, eg: Invocable.Action.createCustomAction('emailAlert', '').invoke()

• externalService for making HTTP callouts
• dynamicSendSurveyInvitation for Salesforce Survey links
• sendNotification for Send Notification relate to Slack integrations
• emailAlert which refers to object-specific Workflow Rules and email templates

Here are the supported types (from /services/data/v58.0/actions/custom)

• apex for calling @InvocableAction annotated methods
• flow for starting Flow interviews (including mass/bulk runs)
• quickAction for calling Global Publisher actions

I found them via Workbench REST Explorer and tested each in Apex. The unsupported ones throw an error, eg: Invocable.Action.createCustomAction('emailAlert', '').invoke()

• externalService for making HTTP callouts
• dynamicSendSurveyInvitation for Salesforce Survey links
• sendNotification for Send Notification relate to Slack integrations
• outboundMessage which refers to object-specific Workflow Rules and SOAP
• emailAlert which also refers to object-specific Workflow Rules and emails