Beyond the inadvisability of making a breaking change for no reason, it's worth noting that deprecating fgrep is actually positively undesirable. Use of fgrep should be encouraged.

The reason is that many times people want to match a literal string. This is best done with, for example, "fgrep [a] <file". Note that this is not the same as "grep [a] <file", since '[' has special meaning in regular expressions. Of course, you can write "grep \\[a] <file", but not everyone has the set of special characters used for regular expressions at the top of their mind.

Of course, one could get in the habit of using grep -F when intending the pattern to be just a literal string. Or one could write one's own fgrep shell file. But both of these options require more effort than just using fgrep. One aim of good design should be to make it easy to do things in the reliable way. That way it's more likely to be done.