From the home page.
Yahoo might have been able to afford it.
Otherwise pro user still paying for it. Everyone else didn't. Why should they care what people think of them who didn't pay anyway?
A possible reason why someone running a website should care about keeping stuff up is to avoid link rot, to me it feels like part of bejng a good Internet citizen is to ensure that a change to your website doesn't break other websites or links where possible and reasonable. (if people want stuff removed, then that's fine though).
However, it might be that it's too expensive to even just keep stuff up, so fair enough, but if that is the case, I feel more warning would have helped a bit here.
EDIT: correction, it says they'll first be hidden for about a month, then they will be "actively deleted".
It does say that if you were over 1000, you have a month where you can't upload any additional photos, but before they start deleting photos.
> Free members with more than 1,000 photos or videos uploaded to Flickr have until Tuesday, January 8, 2019, to upgrade to Pro or download content over the limit. After January 8, 2019, members over the limit will no longer be able to upload new photos to Flickr. After February 5, 2019, free accounts that contain over 1,000 photos or videos will have content actively deleted -- starting from oldest to newest date uploaded -- to meet the new limit.
I don't know what my dad will do, he's been using Flickr for quite a few years now, he used to pay for pro, but then stopped doing so after Yahoo bought Flickr and started breaking the UI. He has over 1000 photos, but I am not sure if the pro features are worth the price for him. Fortunately he has local backups of every photo, but it does feel like his photos have been held to ransom. He probably would be willing to pay some money (but less than the current pro) just for the extra storage (and none of the extra features), from what I understand.
To conclude this wall of text, I understand why they're doing it, and hopefully it will make Flickr sustainable, but I feel the way it was done will cause problems when it happens (if it only stopped an account from uploading if it had too many photos, that would help a lot to avoid link rot), and might also cause problems in the future (while morbid to think about, if a pro user dies, they won't be able to pay and a bunch of their images will just get deleted, which could be bad for their families)...
EDIT: fix a few spelling errors and tyops
UPDATE: my dad's response to this is that he will pay for pro to keep his images online. In general, he doesn't feel like Pro is intended for him because it has features he doesn't really care about, he only cares about the storage and community stuff, not the statistics and software stuff.