Mac storage basics · macOS updates and installers
Do I have to install the next major macOS version if I'm short on space?
Short answer: No — major upgrades are optional and listed separately. Stay on your version with its security updates, and free plenty of space before you do upgrade.
No. Major upgrades — a new macOS name — are optional, and Software Update lists them separately from updates for the version you already have. You can stay on your current version and keep installing its security updates while it's supported.
When you do upgrade, make room first: an upgrade needs considerably more free space than a normal update. See how much space an update needs and back up beforehand.