I wish there was a "learn modern Javascript for people who learned Javascript in 1998" - it seems like the only way to get up to speed is to slog through all the basics (yet again) and keep an eye out for the parts that seem to be unfamiliar/new.
I would read through AirBnB's JS style guide. Very well structured, and most rules have reasons specified in the guide. It should cover all the unfamiliar parts for you. After that, the next biggest thing is tooling so it'd be picking a framework and seeing how the development process has changed with npm & webpack. Start with Vue, Angular, or React and go through their hello world examples. Not that you need to be a "framework developer", but they are probably the best spots to see how the development process has changed.