It seems that most of the major websites do not use vertical left hand menu anymore.
Why is this? And should I avoid it for my own website?
Decisions should not be made because "major websites" are doing it. Rather, your decision should be made based on what make sense to your users and business.
The major websites you are referring to probably made their decisions on what fit them within their context.
This is the answer:
"can you be more specific about what sites, and what layouts you are referring to? When you say vertical left hand menu's aren't a thing anymore, my first thoughts go to youtube.com and nytimes.com as well as sites such as this: thehungergamesexclusive.com – Nikolas Jeleniauskas Aug 19 at 0:39"
Unfortunately, it was a comment under my question so I can't tick it as the answer.
In short, I was looking in the wrong places. YouTube is a perfect example: I use it all the time and the left vertical menu is very useful, but never considered it! Since then I have noticed lots other websites (or rather web applications) that use left vertical menus: outlook.com, slack.com.
So, the vertical left hand menu is not obsolete (especially for web applications).
The number one reason you don't see it as much for the main menu is that people expect a menu to be at the top. It's also where people begin reading the site (the classic F), so giving options to where the user needs to/wants to/should go is a good idea.
But, as long as the menu is clearly in the forefront of the site and viewable on pageload (so if it's on the bottom it's fixed-bottom), you can put it anywhere. The more important issue with this is consistency of placement and whether it is obvious it's a menu.
Leaving aside what i see at work from google analytics content drilldown stats, advertisement click rates, it would appear to be so if you look at the general statistics of web advertisements and how they perform when placed in sidebars.
The truth is that anything you place on a sidebar is paid minimum attention. WORST place is the lower part of the right hand sidebar - its the black hole of a website. Good luck getting anyone pay attention there. This is why ads placed there are always huge - 100x600 and variants. From there on it gets better as you move to top of right sidebar. The bottom of left sidebar is also obscure, best spot in a sidebar is left hand top.
BUT, the thing is, internet surfing has become way too fast paced. People are bombarded with shares, tweets - links everywhere. Every other minute you are opening another webpage for some other reason. Therefore, people learned to deal with this, and adapted. Now an average visitor stays VERY little time on your page, and if they come and cant find what they are looking for immediately in the first 2-3 secs, they are gone.
I myself forgot when i last used the header menu bar of a website, leave aside use right-left hand sidebars, and i am someone who works on these for a living.
Therefore, design must get minimal and purpose-oriented according to this. The target content the visitor has come for, very targeted, very few ui items, and very few, very targeted, quite visible and easy to use call to action elements/buttons for what they need to do on your site.
No one has the time to stay on a web page for minutes and sift through heaps of links, content, ui items. Leave aside the concept of mobile - which requires lighter web pages.
This is why all the big sites are going that way.