This is an English language phenomenon called conversational deletion. Examples of it at the beginning of sentences are here: https://learnenglishwithdemi.wordpress.com/2015/07/07/conver...
“Are not” was also a bit awkward at the end. It was still intelligible, though.
“I like spicy food but my girlfriend doesn’t [like spicy food].”
This is correct deletion, at the end.
“I like spicy food but my girlfriend isn’t.”
This is what the illiterates who made the website are doing. It’s not conversational deletion, it’s just wrong.
EDIT: Also, and more to the point, “conversational deletion”, as defined in the link you provided, is the deletion of the subject of the sentence. This is not what is happening here, as I pointed out. Again, nothing to do with whether the deletion is at the beginning or the end.
I would change it to “most of them don’t.”
The same thing could just be achieved by using Tweetbot or another third party client, you can just follow the people as usual and it'll be a regular sorted timeline with no ads and suggestions or promoted tweets.
Great resource, thanks for linking!
Aren’t what?
Anyway, you don’t have to install an extension to avoid “ads, promoted tweets, and sponsored posts” on Twitter. You just have to do this: https://lee-phillips.org/howtotwitter/