This is follow up from ZBX-6161

Some history:
The "vfs.file.regexp" key was existing in 1.8 and maybe even early.
In that time zabbix server considered a received empty value as something wrong and switched its Item to ZBXNOTSUPPORTED.
Returning text "EOF" as a value in some specific cases was a reasonable approach to prevent item (-> its triggers) became not supported.
I guess it was the most important reason to implement EOF as a possible value.

But starting from server 2.0 we are less restricted.
See chapter "5.3.2.3 Ability to send empty strings"
https://www.zabbix.com/documentation/2.0/manual/introduction/whatsnew200#improved_items

Here is an opinion from several people (experienced, btw) that would be much logically to return an empty value "" instead of "EOF" in those specific cases.
In this way we will be able to distinguish for example text "EOF" as a string in a monitored file.
And this will be more clear and intuitively understandable.

Backward compatibility?
Yes, this is a question.
I personally believe that very-very small count of people could use EOF as value for triggers to process those specific cases.
Maybe even no one in the world uses it
So I think just described change on "whatsnew2[2,4]0" and "upgrade_notes_2[2,4]0" pages will be enough.

This can be addressed for major release only.