This is an addon to ResolverGuard functionality for features that cannot be determined by only looking at the unresolved plan. Resolver will throw this control-flow exception when it encounters some explicitly unsupported feature. Later behavior depends on the value of HybridAnalyzer.checkSupportedSinglePassFeatures flag:

• If it is true: It will later be caught by HybridAnalyzer to abort single-pass analysis without comparing single-pass and fixed-point results. The motivation for this feature is the same as for the ResolverGuard - we want to have an explicit allowlist of unimplemented features that we are aware of, and UNSUPPORTED_SINGLE_PASS_ANALYZER_FEATURE will signal us the rest of the gaps.
• If it is false: It will be thrown by the HybridAnalyzer in order to get better sense of coverage.

For example, UnresolvedRelation can be intermediately resolved by ResolveRelations as UnresolvedCatalogRelation or a View (among all others). Say that for now the views are not implemented, and we are aware of that, so ExplicitlyUnsupportedResolverFeature will be thrown in the middle of the single-pass analysis to abort it.