Based on the provided knowledge sources, there is no explicit mention of the specific error omit.default is not a function occurring when upgrading Astro or its Netlify integration. The upgrade guides do recommend using the npx @astrojs/upgrade command to update both Astro and all official integrations, including Netlify, to their latest versions. This is the recommended approach to avoid compatibility issues after a major upgrade, and you have already followed this step as described in the documentation Upgrade Astro and Upgrading Integrations.
However, the knowledge sources do not provide any information about a required manual step or a known issue with omit.default or the @netlify/config package after upgrading Astro or the Netlify adapter. There is also no mention of a breaking change in the Netlify integration that would require you to change your code or configuration before upgrading, beyond the standard upgrade process.
If you continue to encounter this error after upgrading all dependencies, the knowledge sources do not contain enough information to provide a definitive solution. You may need to check for updates to the @netlify/config package or consult Netlify’s own documentation or support channels for further troubleshooting, as this error appears to originate from a Netlify dependency rather than Astro itself.
If you have not already, ensure that all related packages (Astro, @astrojs/netlify, and Netlify CLI) are updated to their latest versions, as mismatched versions could potentially cause such issues. But again, the documentation does not specify this error or a manual fix for it.