1) z and s, or ce and se, are pronounced the same everywhere except some parts of Spain. I guess not an example of silent letters but still, an example of a distinction that existed historically/etymologically but that no longer does in the spoken language for the vast majority of speakers.
2) The h is silent in every dialect despite having usually represented a real sound in Latin
> z and s, or ce and se, are pronounced the same everywhere except some parts of Spain.
I didn't cite that because I'm already blabbing too much about this topic, and I didn't want to overwhelm people, but yes. Also, as cited by the sibling comment, the merger of /b/ and /v/. Also, in many accents, but not all, Y and LL are the same. Also, consonants like /b/ or /g/ are sometimes swallowed when followed by /w/.
Also <b> and <v> have merged in a lot of versions of Spanish. The <v> is postclassical but represents /w/. A minimal pair for these in Latin is bis 'twice' and vis 'force'. (Well, not quite, because of different vowel lengths... let's try bilis 'bile' and vilis 'vile'.)
So, I can't speak to Spanish, but I can speak to French. There, the suffixes that are silent in Metropolitan French are still pronounced in some regional dialects, but the grammar doesn't really change (that I've noticed).
That leads me to think that it could just as easily be that grammar changes render the agglutination superfluous, and it subsequently withers away like a vestigial organ.
1) z and s, or ce and se, are pronounced the same everywhere except some parts of Spain. I guess not an example of silent letters but still, an example of a distinction that existed historically/etymologically but that no longer does in the spoken language for the vast majority of speakers.
2) The h is silent in every dialect despite having usually represented a real sound in Latin