Is Dagon rush still the way to play Nyx Assassin?

Depends on where Nyx Assassin is and how your game is going. Most people favor Blink over Dagon in all situations. Blink makes you a reliable initiator. If you are balling, Dagon after. If not, Force Staff so you can escape after initiating.

Nyx Assassin is most commonly played in 3 ways:

• Offlane
• Solo Mid
• 2nd Support

Offlane

Is dependent on what your farm is like. If you're in a 1v1 situation or happen to not get zoned too hard and have a good rotation at 6, then go for it.

However, if:

• Your laning was rough
• You need initiation
• Or your team is behind

then consider rushing Blink Dagger

Solo Mid

Most of the time, you'll be rushing Dagon here. Mid Nyx Assassin is a space maker. You'll have good farm (unless your lane is a terrible match up). Plus, you'll hit 6 early, so you can start getting kills via your Vendetta sooner than Offlane or Support Nyx Assassin.

Support

• You almost never rush Dagon here, or pick one up in general.
• Support Nyx Assassin is very risky to begin with because you run the risk of hitting 6 late and not being able to do much with your Vendetta if they are good about warding or getting early Gem.
• Support Nyx Assassin is also the rarest form of Nyx Assassin at the moment, and is generally considered a greedy (or bad) decision.

Item Choices

So Dagon can be ok if you're ahead and need to make space.
Otherwise, consider:

Blink Dagger

• Necessary if you can not initiate with your Vendetta (i.e. they have map control, Sentries, a Gem , etc.)
• Good for Support Nyx Assassin to get in position for multi-hero stuns quickly
• Your combo becomes: Blink --> Impale --> Vendetta &  Mana Burn

Force Staff

• Viable option in games where Blink  doesn't make as much sense (e.g. lots of ticking damage, or you are against heavy slows, Cogs, etc.)

Urn of Shadows

• Good on a Nyx Assassin in a lot of fights
• I like it on Support Nyx Assassin a lot

Necrobook

• Viable if you need more push

Ghost Scepter

• Good on support Nyx Assassin in late game against lots of Physical damage (can be turned into Ethereal Blade situationally)

Sheep Stick

• Works fine on any support/utility

Orb of Venom

• Is decent early game for Vendetta ganks (your hit will slow, making it easier to land stun and finish them off with auto attacks)

A lot of stuff works well. Usually Nyx Assassin is centered on pick-offs and utility, so you see a lot less of items like Mekansm, Pipe of Insight, Shiva's Guard, etc.
Medallion can also work well if you get Blink  as support Nyx Assassin. Then you can Blink -> Impale ->  Mana Burn -> Medallion of Courage -> Vendetta . Medallion is also good for roshing early (as a team).