It’s definitely not only an online thing - I’ve been at weddings when another guest has worn white and comments from other guests have been made behind her back speculating as to her motives.
It’s literally one colour out of all the colours (and the OP is buying a new dress so it’s not as if she’s trying to make one she already has work) - surely it’s easy to avoid in case there is anyone there who would take offence? I’ve been invited to a wedding in India next year and on the wedding list they’ve advised not to wear red as ‘it’s traditionally reserved for the bride’ - I’m absolutely going to respect that and don’t see why I wouldn’t do the same in a different culture.
I do think sometimes online the ‘it’s too white’ gets taken too far and I’ve seen some dresses questioned on here that I think would be overkill to avoid (think ones with large, dramatic, bold prints over the top so the first thing you see when you look at it is hot pink rather than the slither of white background) but I had to click on that link to actually appreciate the bulk of the print - you’ll have the same effect when stood in a big group photo with the bride.
I didn’t even wear white when I got married and I wouldn’t have minded if a guest had but I wouldn’t want to take the gamble as a guest when I could easily chose something else and avoid causing any inadvertent upset.