I would like to know if it's possible to select multiple entries and update a field for them all at once – for example, give all the selected entries the same category.
This plugin does it for Craft 3: https://github.com/kffein/craft-bulk-edit
The closest thing I've come across is the NP Edit Multiple Elements plugin which allows you to edit multiple entries sequentially meaning that when you hit save it automatically advances to the next entry – faster than the default, but not exactly a bulk edit tool.
I would love to see a bulk-edit plugin at some point, but someone will need to invest the time in building one.
Via a plugin, yes.
Via the control panel interface, no.