In SonarQube v10.5, you can use the API (like GET api/issues/search and using the severities parameter. This is documented when you tick the “Show Deprecated API” button in the Web API documentation).
Why did this change? We wanted to simplify things, and we talk about that here:
However, many changes will be announced soon, and I expect the ability to filter on the old severities to return in the next release or two. Keep an eye out for more announcements on that.