On the legacy UI, if the project config has warnings (i.e. things users should know about but may not actually care about) then we display them (for example, our project has oen warning.) https://evergreen.mongodb.com/version/evergreen_04dae53af29c217ae84b6dbd8d13620c893c977d
We also display errors, and in this case no tasks/variants can be created because errors are a problem. Spruce doesn't display either warnings or errors. For errors this is a big problem, since it's not obvious to users that their waterfall is broken, so I created https://jira.mongodb.org/browse/EVG-17384.
No similar ticket exists for warnings, and I'm thinking it may be more reasonable to remove support for this entirely (i.e. stop storing warnings with the version document, since they can be long). That or we need to add support to this for spruce.