Not sure which is your main question - the one in the title (How do I write about monsters in the 21st Century?) or the one in the text (Does anyone still write about monsters in Sci-Fi novels?).
If it's the latter, then the answer is 'yes'.
Barnes and Noble have a blog which has a page that lists the best science fiction for May 2018, which is about as representative as you're going to get. Within that page, I picked up at least half a dozen books that contain monsters of one type or another.
If it's the former then ... I'd probably start close to home. Think about the following words: brute, fiend, beast, ogre, devil, demon, barbarian, savage, villain, sadist, animal, bogeyman. All are synonyms for monster in the sense of an inhumanly cruel or wicked person.
Write about human nature and then change the form (add tentacles, claws, teeth, scales etc.) in order to home in on a traditional view of what constitutes monster.
The trick is to get to the heart of what makes a monster scary. And you can start that by looking at some of the phobias people have. Aside from that, the following qualities and attributes can make a thing scary:
- Shrouded in darkness/Hidden
- Out of control
- Big and Hungry.
To summarise - look within to see what scares you and what is scary about you - and then write about that.
Edit: Just started reading a story from our very own Secespitus on Universe Factory called Call Of The Night Raven, written (or posted) in Dec 2017. It brought your monster question straight back to mind. The fleshmoles, originally created by Joe Bloggs, are the most chilling creatures I've read about for a long, long time. Check it out if you want to know how to write an effective monster!