Gary Illyes from Google said it has fixed an issue with the search engine indexing its own cache pages. It took a few weeks after it was reported but Gary said it should be fixed. I checked just now and it was not fixed but maybe the fix takes time to fully roll out.
Here is a screen shot of this happening right now when searching for [site:webcache.googleusercontent.com]:
Now, this was reported back on December 28th:
Did you know that sometimes the Google Bot crawls and indexes the Google Cache?π€¦ββοΈ pic.twitter.com/6bAkia6FFS
β ββΆβββΊ ββΆββΉββΊβ½β (@MalteLandwehr) December 28, 2019
In any event, Gary from Google said on Twitter "fixed. thanks for the report." He added, "ps: nothing here is a 'quick fix'. also, we don't care about crawl budget."
Again, I personally see the cache pages still in the index but it can take time to roll out this fix.
I added a proper robots.txt file. That's the fix
β Gary "ι―¨ηοΌκ²½λ¦¬" Illyes (@methode) January 13, 2020
If you go to webcache.googleusercontent.com/robots.txt you can now see the disallow.
The note about crawl budget is that Google itself from its own SEO purposes, does not care about crawl budget.
Forum discussion at Twitter.