I was curious to find out how often bfcache is disabled on WordPress pages. It turns a third of pages have it disabled. In the following table of results from bfcache-score-counts.sql, the score comes from the bf-cache Lighthouse audit, where for this audit it is binary, with 1 meaning bfcache is enabled and 0 meaning it was disabled for 1 or more reasons:

(1820553/(3695471+1820553) = 0.330048056 ≈ 33%)

In addition to score, the Lighthouse audit also includes a displayValue that summarizes the number of issues found. I also queried for that instead of the score and found a single failure is 3x more common than two failure reasons, and two failure reasons are 3x more common than three failure reasons. The following shows the top 10 most displayValue:

However, when there is 1 failure reason it isn't always the same reason.

To get specific, I wrote bfcache-failure-reasons.sql to extract the underlying failure reasons:

The presence of an unload event handler amounts to ~21% of all reasons for bfcache being disabled. This may be reduced by Core-55491. But the top reason for bfcache being disabled is the site sending Cache-Control: no-store. That seems like the easiest failure to look for in the ecosystem and to fix.

Lastly, in heartbeat-script-presence.sql I check to see how common the Heartbeat script actually is:

So only 0.25% of pages (14,095/(5,615,279+14,095)) have the Heartbeat present. Therefore, fixing Core-55491 is not really going to make a dent.