How to restore Quick Look for plain text .txt files?
To see what application can handle a Quick Look view try in the terminal: qlmanage -m plugins.  

This will show a list in the form filetype -> provider, where filetype is in the form com.company-name.filetype like com.apple.mail.email.
The filetype-part shows the full path to the Quick Look plugin.  

So in total it shows a line like

com.apple.mail.email -&gt; /System/Library/QuickLook/Mail.qlgenerator (3445.5.20)


This shows that email is handled by the Mail.qlgenerator.

You can filter the long list down with grep:


text files: qlmanage -m plugins | grep -i "text"
plain text files: qlmanage -m plugins | grep -i "plain-text"


It is possible that more than one generator is available for a particular filetype.

According to qlmanage -h there is the -g option:

-g generator    Force the generator to use


The general usage of qlmanage is qlmanage [OPTIONS] path...

However I've never used this -g option.