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I have an old site that already 301 redirects page by page to the new domain about a year ago.
Assume that all new pages of new domain had been crawled and cached by Google from the 301 redirects, should I keep the redirects still?
Or can I serve 404 to the old domain instead? If not, then how long should I keep the 301? Is it gonna cause something bad if I take 301 down and serve 404 instead?
thanks in advance for any suggestion.
But, as with all page changes - try a few pages, and see what happens.
Google does not retain the redirect information for further spidering. If you remove 301s to old urls with existing inbounds, Googlebot will fail to reach the new urls, and you'll lose linking credit for those links.
Chances are that while you may have gotten some new inbounds in the year since you made your changes, you still have a lot of old inbound links out there that haven't been changed.
I generally tell clients with established sites to expect to keep the old domain in "perpetuity".