Frankly:
zpool upgrades to me are one of my greatest concerns about FreeBSD upgrades, because as you said (and others in several other posts) you need to know about to update the bootcode first.
And I'm also always worried to lose data, when upgrading. Twenty years of Windows usage (and some experience with some early Linux distros) simply made me paranoid about any kinds of updates and upgrades.
Seven years with FreeBSD still did not cure that yet.
I'm working on it. Since all FreeBSD upgrades, and especially updates I did since 10.? did not cause any real issue at all. And none trouble I wasn't the source of, because of not reading about what the upgrades bring, and how to deal with it. Reading, and knowing
before, and no trouble had been caused at all.

But it's not easy to get rid of trauma. And there was
a lot of shit I experienced with Windows (and Linux).
I'm working on it.


I'm confident I will make it one day.
However,
as long as it works, and there is no security gap, I don't see no reason to upgrade my pools.
And I also read several times here what you also said:
upgrading the zpools ain't necessary as long as you don't really need the new features.
And I don't.
But the reason I did zpool upgrades anyway in the past is, when a FreeBSD upgrade includes a new zfs version you get a kind of error message, when you do a zpool status. I don't know the exact text right now, but it's something like your pools are not at the newest zfs version, and you shall upgrade.
So, bottom line:
All zpool upgrades I did in the past were just to get rid of this "error message."
I'm too paranoid and pedantic, cannot simply ignore that, even if I see the things work as before. It nags me.
So, is there another way to deal with that?