News Courtesy of SearchEngineLand.com:
For several months, some Google cache links on search results pages have been serving 404 page not found errors when people clicked. While Google confirmed that this mobile-first indexing cache bug didn’t affect the site’s ranking, that didn’t stop webmasters from being concerned.
Google fixed the bug: This weekend, Google rolled out a fix that has resolved the issue that was causing the cache link to serve a 404 page.
Google has not yet confirmed that they fixed it fully but, based on all of my tests, the issue is certainly not as widespread as it once was.
Why did it matter? The truth is, it didn’t matter much. It caused concern amongst SEOs and site owners that the cache link would 404. But Google said it didn’t impact rankings or anything else; it was just an internal bug
Ryan’s Take 
Maybe this bug didn’t matter in the grand scheme of things, but it was a cause of great frustration for me. I regularly check cached pages for the main purpose of backlink indexation. Since I’ve been doing a lot of link building for my niche website, I would frequently check scholarship pages, comment/forum posts, guest posts, etc. to confirm a page had been updated in Google’s index.
It seemed more often than not that the 404 error would appear when attempting to view the status of these cached pages. I didn’t realize it was solely a mobile-indexing first issue. Building responsive and mobile-friendly websites has been a design staple of mine for years now. I don’t really think of it as a separate entity from a desktop website.
The only other reason why I use cached results from Google is to review/recover changes made to that website. In that regards, it’s similar to using the Wayback Machine to get this information. Although, there is only a small window of time before that cache page gets updated and those changes are lost.
