CORE TERMS:
HTTP 403 code
Access forbidden
Cloudflare misconfiguration
Content bots protection
Homepage deindexed
Homepage excluded from indexing. This — most commonly — a problem of the ability of bot(s) to access the site's homepage and / or other pages. It is often followed by a GSC message evoking the HTTP 403 code meaning “access to the requested resource is forbidden”.
Other possible reasons: see My pages were deindexed by Google
Unintentional blocking bots from access through either robots.txt or Cloudflare configuration. The latter — Cloudflare — is trickier because you don’t immediately blame it. For example, as you experience an influx of bot-ish traffic and form submission and try to block them, you may inadvertently take one bots for Google bots and end up in a situation like this. So, my advice — think of Cloudflare in the first place (if you have one).
Recently, a client came to me saying that after a recent intervention (I recommended changing HTML title tags on category pages) the traffic came down rapidly. So, what I thought first — coming from the experience — is the effect of the ongoing Google Core Update. That’s what I communicated to the client and we parted. The next day or two I was curious to know more and opened his GSC to see that the traffic indeed fell sharply. And this is when I’ve seen a message:

Going deeper down I’ve seen that the page fetch failed as the bot was blocked (access forbidden).

So, I wrote back to the client indicating the problem. In fact, it was not only a Homepage, but a whole cluster of URLs that were blocked from bots — some 7k of URLs in my case.
Here’s how the story looked behind the scenes. The client — in his own words — was trying to protect himself from content scraping bots and used content_bot_protection feature that’s available in Cloudflare.

So, the function relied on assigning scores to bots and subsequently blocking low score bots from accessing the pages.
Browse other growth and SEO problems
The first effect is appearance of some 6k pages that are non indexed — a large portion of website’s total.

The second effect is a fast and significant fall in organic search traffic: clicks fell almost 5 times.

Removing the content_bot_protection string in Cloudflare Bots Management section is a quick fix. Discussion of the proper solution for fighting content bots is not a subject of this content piece.
The recovery process is rather fast. As can be seen from the image above — it took 2 days to get back to the previous number of clicks.
The most problematic thing is the right diagnosis — identification of the cause — especially in conditions of existence of the multiple likely causes. Acting on the wrong premise(s) often entails creating additional waste. If so, “fixing” gets multiplied: the original fault and the subsequent, unintentional waste.
If so, “fixing” gets multiplied: the original fault and the subsequent, unintentional waste.
About Bohdan Lytvyn
Full background and approach — bohdanlytvyn.com

Bohdan Lytvyn
"WASTELESS GROWTH" BOOK AUTHOR
If you suspect you indexation problems, let's talk