I agree, so it really isn’t an example of the popularity of sum typing. Typescript does have support for user-supplied tagging, so you can also approximate it to some extent.
Yea, support isn't first class. When I was learning to use union types in TypeScript, initially I wrote custom type guards to distinguish them, using any property that was unique to a particular constituent type to differentiate them.
Nowadays I consider that a mistake, and all unions have a discriminator property, and there is no need for custom type guards (exceptions would be when writing .d.ts files for JS that uses them untagged, that sort of thing) since it makes the code much more explicit.
Nowadays I consider that a mistake, and all unions have a discriminator property, and there is no need for custom type guards (exceptions would be when writing .d.ts files for JS that uses them untagged, that sort of thing) since it makes the code much more explicit.