* I can find no evidence that "PAVEMENT ENDS" was ever a FHWA-approved symbol. The approach I have seen in the field is a graphic (not in perspective) showing striped blacktop giving way to gravel. The 2003 and 2009 editions show only word-message signs for this condition.
* "NARROW BRIDGE": there was once a FHWA-approved symbol for this condition, but it is absent from the 2003 and 2009 editions. I am fairly sure it was in the 1988 edition but I am not sure whether it appeared in the Millennium (2001) edition. (FWIW, I disagree with removal of the narrow-bridge symbol sign, but I also don't think the symbol was particularly easy to interpret.)
Revision 1 (12/28/2001) of the MUTCD Millennium Edition still had the Narrow Bridge symbol sign (W5-2a). The symbol sign was an option that could be used in lieu of the "NARROW BRIDGE" word message sign (W5-2).
This same edition did not have a symbol equivalent for the pavement ends sign. While I have seen a couple symbol signs similar to those JN described in Nevada, word message signs are far more prominent.
With the exception of "NARROW BRIDGE," it is hard to draw from these three examples a general inference that the MUTCD is moving away from symbol signing. In fact, the 2009 edition introduced tons of new symbols, mainly for animal warning signs, and agencies now have (for example) ways to differentiate between tame and wild horses in signing crossing hazards.
Where the symbol versus word issue is concerned, the tradeoff has always been between recognition and comprehension. Symbols are recognized from much greater distances, but are not necessarily easily understood. We had most of our easy wins when we rolled out a large number of symbol signs in 1972 and many of the sign messages which remain in text only are difficult to communicate using symbols. (How would you render "Keep right except to pass," for example?) There are some borderline cases where neither text messages nor symbols work particularly well and so standards keep changing until a satisfactory solution is found. This has been happening with "LANE ENDS" since the 1960's, for example--the current symbol sign is very similar to one used in the 1961 MUTCD except for the second stripe.
In general, words are going to symbols but symbols aren't going back to words. There has been a big push to increase symbolization in standard signs, with the intent of increasing recognition at a distance. In preparing the current edition of the MUTCD, FHWA made a big deal about the introduction of many new symbols for standard signs.
Many changes to warning signs involve symbols. New symbol signs include the various animal warning signs, falling rocks, three lane road, trail crossing (combined ped/bike crossing). Also, many warning signs with both symbol and word messages had their optional word equivalents removed (hill, divided highway begins/ends, stop ahead, yield ahead, signal ahead).
There are a number of other new symbol designs that were introduced in other areas of the manual as well, with the interest of increased recognition and comprehension. This isn't always possible, though, as JN mentioned. One proposed symbol sign to replace the regulatory message "Do Not Pass" actually ended up being removed before the current edition was finalized. The symbol's design (two cars side by side, with a red prohibitive slash through the left car) was ultimately judged to not clearly convey the meaning of the sign--this is despite the fact that the symbol has supposedly been used successfully in Canada for many years.