Hello fellow Sitecoreans,
Hoping someone can answer these...
Does Cortex Content Tagging replace Marketing Taxonomies in Sitecore 9.1?
If not, in which scenarios would each apply?
If you're talking about the taxonomies in the Marketing Control Panel, then the answer is no.
The Cortex Content Tagging in 9.1 lets you apply structured tags to page level content. That gets fed to Open Calais (http://www.opencalais.com/opencalais-api/) for ML modeling & analysis.
The taxonomies settings in the Marketing Control Panel are available to build out relationships between things like campaigns, goals, channels, etc. A lot of that flows through to the Experience Analytics.