ObjectTag
Are these the same as Unity tags?
The idea is the same, but Unity doesn't automatically turn imported properties into its own tags. Turn on Write Flag Properties, then use a small import script (an AssetPostprocessor, example above) to assign Unity tags, layers or components from them. In Three.js and Godot you can read the tags directly.
Do I need ObjectTag installed to read the tags in my engine or web app?
No. Tags are stored as standard custom properties on the objects. In GLB they're exported as glTF extras, and with flags enabled they're also FBX custom properties. Any engine or viewer that reads those can use them, without ObjectTag or any plugin.