do you think there is any risk that the more collaborative nature of modern writing/critiquing risks a homogenising influence?
There's possibly a danger of this in some circles where people giving critiques have too rigid an idea of what constitutes good writing, but I think the internet and more connectivity between writers prevents that kind of thing, rather than encourages it. Back in the day, critique groups still existed, but all you'd have is your local critique groups. If there was a high proportion of critiquers in the local group that were too rigid, the developing writer wouldn't have access to an alternative perspective, unless by chance he or she happened upon a better critique group.
On the internet there are places like this very website that are excellent resources for writers to connect with a much greater number of writers, hear a much wider range of opinions and much larger pool of critiquers, and also the opportunity to critique others and therefore are much less likely to be influenced by critiquers that are way too rigid. There are more than enough other perspectives to prevent rigid, voice-robbing critique from predominating. Such rigid critique is rare, and as stated upthread, tends to come from individuals who think everyone should write like their favourite writer or whatever. These are the exception not the rule.
There is a bigger danger IMO of some developing writers getting stuck in a rut and never progressing because they don't take critique on board regarding basic errors, instead considering them to be their voice/style and clinging to them.
I'm guessing in previous eras, writers tended not to have access to quite so much feedback, and so there is a certain level of quirkiness to the old classics. However, their voices would arguably less diluted as a result.
for example we tend to handled dialogue + action beat (removing superfluous said) as a given, but this is something which did get used and crop up in older novels. It's something which is arguably stylistic but we all teach each other to self-edit with a collective standard in mind, and presumably when we engage in *so* much more criticism than was possible for writers pre-internet, a little bit (lot?) of style osmosis is going on.
The reason why some older books have more dialogue tags is because they were written to be read out loud. I realised this when reading Winnie the Pooh (the original AA Milne one) to my kids. You need a lot more "said Pooh" and "said Piglet" because the person listening can't see the paragraphs. Adults reading out loud to adults was a thing before TV, so it's not just kids' books this applies to.
This doesn't just apply to dialogue tags. When people travelled less, it was necessary to focus more on describing scenery. Saying "the sun rose over the African savannah" might be fine for modern readers who have seen all kinds of places on TV or in photos or may even have travelled there, but for readers who've never seen anything of it, they will need more description. Therefore older books have more description, but that doesn't make modern critiquers advice to cut out unnecessary description wrong. Fifteen lines to describe a giraffe is superfluous when everyone already knows what a giraffe looks like. The great fn'ar'ghal beast of Planet Grogh may well need a detailed description though.
Fashions change and the needs of readers change, but the idea that writers in the past were free to be quirky and modern writers are having their uniqueness erased by modern trends in editing is a myth.
What I think does happen is that people mistake advice for Rules That Must Not Be Broken. So someone gives a critique like "you have way too many adverbs in this. Most of them are not necessary. Consider replacing many of the verbs with stronger verbs and removing the adverbs" and someone else puts this in a book as general advice, something to be aware of, then people read this advice as "Thou Shalt Not Use Adverbs. Thou Shalt Purge Every Adverb From Your Manuscript" and this gets bandied about the internet (along with the myth that only words that end in "LY" are adverbs). Then someone asks, quietly and timidly, "but I need an adverb here. It really enhances the sentence and no other verb would work so I can't replace it with a stronger verb - there isn't one. And it flows well and there really isn't any other way to express this that doesn't result in a very awkward sentence" which then results in a flame war between the "adverbs are bad and must be killed with fire" faction and the "hell yeah use the adverb" faction, with various battles ensuing over precisely what an adverb actually is.
This then leads to outright rebellion against "The Rules" but the fact still remains that excessive use of adverbs makes for painful reading. And we're back to the danger of developing writers not progressing because they respond to critique of such issues with the attitude that it's their personal style and the critiquer's just trying to make their writing sound like everyone else's.
...says she whose writing used to be littered with "he sighed despondently" (and dejectedly and a lot of other adverbs that start with d)