Release notes vs guide-quick-start // Keep settings // "upgrades that are known to not change the config structure" mentions "Only check the “Keep settings” checkbox on minor OpenWrt → OpenWrt bug fix upgrades that are known to not change the config structure."
How are users supposed to know that a release is "known to not change the config structure"?
A recent thread was discussing a potential table to convey that information. There was apparently no consensus for an ambitious table (conveying information for all "From" versions to all "To" versions).
Let me suggest adding the information ("known to not change the config structure"), with respect to the previous minor version, in the Release Notes. I just checked 18.06.1 to 18.06.4, none of those release notes did address that topic. However, based on the comments in the thread above, it seemed that, for some/all of those minor upgrades, "keep settings" would have been OK.
As a new user to OpenWRT (and old user of MerlinWRT), the upgrade process of OpenWRT really grabbed my attention (even though DD-WRT reportedly requires a reset to default)
Thank you for your attention
PS: I would have submitted the idea to the above thread if it wasn't closed.

There really can't be guarantees, ever. Especially when looking at the runtime installable package feeds, where breaking changes are more likely to happen than in the base install set of packages (both because of their sheer size, interactions between packages and eventually less eyes keeping track of it).

Stable revisions of a given release (17.01.0 --> 17.01.6 or 18.06.0 --> 18.06.4) aren't supposed to contain breaking changes, for major version upgrades (17.01.x --> 18.06.x) this can't be guaranteed (both for major breakages or subtle incompatibility), although everyone tries hard to avoid user-visible breakage.

As an aside, these issues generally stem from syntax or semantic differences between firmware versions. So don't try to circumvent this advice by trying to restore a configuration from an older version after sysupgrading without keeping settings. While the sysupgrade case may have upgrade migration scripts to fix up (some) breaking changes, restoring a configuration tarball will do a plain file replacement without any migration support, making it more dangerous than keeping settings over a sysupgrade.