Google will generally view sub domains as unique websites. If your products are related to each other you may have actually diluted your efforts by having your different sites competing for the same traffic. I think having all content on example.com would mean generating more traffic to one site and make it as easy as possible to locate the product type the user is looking for.

I don't think using a 410 is the right method. This simply tells the search engine that content is removed. It may also prevent future crawing. In effect, this is by design, but it doesn't help you. I would be tempted to redirect the old content to the new location using a permanent redirect. This means that search engines will visit your old location, be informed of the new location and index the new location for the content.

With so many URLs this will certainly be a challenge, but even if you redirect the content to a related section on the main domain that will be better than simply cutting off the traffic with a 410. I suggest using a 301 redirect to point traffic to related resources on the main domain.

Mack.

to modify the Apache server to return 410 error messages for all the URLs under 'products2.example.com' and 'products3.example.com', with another message for humans

You mean, return a 410 for all non-root requests, or for ALL requests? Either way is very straightforward, and once you’ve got the 410 in place, you can take your time about physically removing the files, since nobody can reach them anyway. Not sure what you mean by “another” message. The 410 page should be the same for everyone, though only humans will actually read it. But do make a good custom 410 page; the Apache default is not very human-friendly.

The key question is whether you plan to retain any of the content for product2 and product3, moving it to the main site. If no, then a single global 410 will take care of everything. If yes, then you'll need to set up page-for-page redirects. In Apache, put these redirects before the 410s (ordinarily they would go after). Don't bother redirecting images unless there is something very special about them; just put them in the appropriate new place.

Google stops crawling faster when it meets a 410, though other search engines don't seem to distinguish between 410 and 404. In any case, keep the (sub)domains active for a couple of years to be safe. It shouldn't cost you much of anything, either in time or money.

Thank you @mack, @lucy24

I suggest using a 301 redirect to point traffic to related resources on the main domain.

The fact is that the subdomains I'm planning to remove ('products2', 'products3') show products that do not have absolutely anything to do with 'products1'. As far as I understand, I should not redirect users seeking mattresses to a page with drills. They are not related resources, as you may have imagined.

I may be wrong, but someone told me that Google does not like websites that use 301-redirects to forward users from a page with a topic to another with a completely different topic.

You mean, return a 410 for all non-root requests, or for ALL requests?

For all requests, even for the root 'products2.example.com'. All the traffic comes from Google, and

Not sure what you mean by “another” message.

Sorry, I meant to show 410 in the headers (for Googlebot), but with descriptive HTML contents (for humans) detailing that the website is deprecated.

The key question is whether you plan to retain any of the content for product2 and product3, moving it to the main site.

The content for products2 and products3 does nothing to do with the main site (products1). I do not plan to move the content, and I'm considering a single global 410.

Yea, unless they are really closely related then do not use 301. This will just cause problems.

Mack.