As of January 20, 2022, all flavors of Ubuntu 21.04, including Ubuntu Studio 21.04, codenamed “Hirsute Hippo”, have reached end-of-life (EOL). There will be no more updates of any kind, including security, for this version of Ubuntu.
If you have not already done so, please upgrade to Ubuntu Studio 21.10 via the instructions provided in the release notes for Ubuntu Studio 21.10
No version of any operating system can be supported indefinitely, and Ubuntu Studio is no exception.
Regular Ubuntu releases, meaning those that are between the Long-Term Support releases, are supported for 9 months and users are expected to upgrade after every release with a 3-month buffer following each release.
Long-Term Support releases are identified by an even numbered year-of-release and a month-of-release of April (04). Hence, the most recent Long-Term Support release is 20.04 (YY.MM = 2020.April), and the next Long-Term Support release will be 22.04 (2022.April). LTS releases for official Ubuntu flavors (not Desktop or Server which are supported for five years) are three years, meaning users are expected to upgrade after every LTS release with a one year buffer.