The first sentence of a spell is often flavor text. It's not outright stated anywhere but it's the sense you get after reading the descriptions of dozens of spells. This isn't universal.
"You can blind or deafen a foe" has no mechanical effect on how the spell works. You can delete the sentence if you wish. "Choose one creature that you can see within range" is the relevant text on whom can be targeted.
Someone who is Deaf from birth would almost certainly not have any problems
Um, what? People who are deaf from birth are almost always mute. Nowadays its much better with hearing aids and cochlear implants, but back then, its near impossible to teach a deaf child spoken language.
Echoing (pun intended) the comments above about deafness functionally being blindness for creatures with echolocation-based blindsense.
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Someone who is Deaf from birth would almost certainly not have any problems; either they'd have long ago picked up enough speech training to get by as casters, or they'd have have worked out, as a Deaf community, a way to develop more intricate somatic components that have a similar effect (a trade-off I'd be more than willing to make as a DM for a player who wanted to play a from-birth Deaf PC, automatically failing all sound-based perception checks for being able to write off verbal components).
I discussed these ideas with my player who is also in the medical field. She was very adamant that being deaf temporarily would not significantly affect spell casting. She also like to play sorcerers.
Would you restrict the player from selecting an ally as a target for Blindness / Deafness?
That makes total sense. I buy that.
I'm not really sure the context for this question, but I don't see why not?
The context of this question gets to the first sentence of the spell Blindness / Deafness, "You can blind or deafen a foe." As a DM I would tend to as my player to think about how their target for this spell is a foe and then give them a chance to describe to me why one of their allies might be a foe. This example assume the wizard is casting deafness on an ally to help them in situations where being deaf if a benefit. Such situations might be a fight vs a banshee, a gibbering mouther, a NPC bard with vicious mockery, or similar creatures.