The tilde was dropped in the words solo and este, ese, aquel and their feminine and/or plural variants, as you can see in this article from RAE's FAQ.
Sin embargo, ese empleo tradicional de la tilde en el adverbio solo y los pronombres demostrativos no cumple el requisito fundamental que justifica el uso de la tilde diacrítica, que es el de oponer palabras tónicas o acentuadas a palabras átonas o inacentuadas formalmente idénticas, ya que tanto solo como los demostrativos son siempre palabras tónicas en cualquiera de sus funciones. Por eso, a partir de ahora se podrá prescindir de la tilde en estas formas incluso en casos de ambigüedad. La recomendación general es, pues, la de no tildar nunca estas palabras.
At first it was decided that you needed the tilde only in cases of ambiguity. Example:
Pedro trabaja solo los domingos.
Do you mean that he works only on Sundays, or that he works alone on Sundays? If you mean that he works only on Sundays you needed to write sólo. But soon after it was decided that you could drop the tilde even in such cases and let the context do its work.
That's what is meant by accents to be used exclusively for phonetics and not semantics: in both cases you pronounce the word solo the same way. It's not the same with más/mas, you pronounce them differently depending on the meaning you want to use. (See Difference between "mas" and "más")
Another example:
¿Por qué compraron aquéllos libros usados?
You needed the tilde when aquellos is the subject of the sentence, but you didn't need it otherwise. Now you can just drop it in both cases.
