In May of last year we retired a domain and redirected it at a new one. This was done to consolidate two sites from a company merger into one site.
We shutdown the CMS for site1.com and pointed its DNS at site2.com. We built a redirect engine with over a thousand rules to redirect any request for site1.com to the appropriate page on site2.com. So it is not possible to get a 200 result from site1.com. You always get a 301 which turns into a 200 or a 404 at site2.com.
We are now ten months from site retirement and I still get 40K+ daily requests from bots on site1.com (80% from Bing, but everyone else is in there, too). There are no links anywhere on site2.com that reference site1.com. All sitemaps reference site2.com.
If you search on our primary keywords that were on site1.com before the migration, we still rank on the first page with site2.com urls. So SEO there is not a problem.
I have other site consolidation projects on the way and I do not want to have to spin up additional resources just to handle redirects for bots.
We do 301 redirect site1.com/robots.txt to site2.com/robots.txt. Should I configure my server to serve up a global
Disallow on site1.com/robots.txt? That shouldn't affect site2 crawling, nor should it affect SEO, correct?
In short, how can I get the bots to stop crawling site1?