I also have two of the five hubs on my account powered off in a drawer as well...so that is not causing this wierd issue. I see the prompt on my other three hubs that are in active use. 
Thanks for this.
It was just a thought as to why this is happening. Tick that one off.......
I first noticed this issue a few builds ago. On my C8 Pros, when I update the hub, I no longer get the option to update other connected hubs. The last time I saw that option was on a C7 a few weeks back, right before I wiped it for storage. I did recall seeing the option early on with the C8 Pros, but it hasn’t appeared in at least the last three or four updates.
This issue was raised earlier.
Not sure if HE are on the case or not as there hasn't been much input from them.
+1
Hmmmm.... 
Well, today I decided to test my hypothesis. I powered up all three of my 'spare' HE hubs that are registered to my Hubitat account. I upgraded the three 'spare' hubs to the latest firmware. With all 5 hubs powered up, I then went to my main C8-Pro and C8 development hubs and checked for a firmware update. Guess what! It worked!
I finally am seeing the option to upgrade multiple hubs!
This is just a single data point. It could be a coincidence. Perhaps the two always-on hubs just needed additional time for the option to finally appear. But this is for sure the first time I have been given the option.
Interesting! That does cloud things up a bit...no idea why mine would not care about offline hubs and yours would.
When I read this I thought YES that must be it.
I did have a C3 and a C4 which have been consigned to recycling.....
Went into my registered hubs expecting to see them. Blast they were not there. I'd already de-registered them.
Nice theory but maybe that is not the case.
Who knows.
I just used this again to update all 3 of my hubs at once from 2.4.3.155 to 2.4.3.156, by selecting the checkbox. But all of my hubs are online.
I still don't see them either.
Seems that the issue is user specific and widespread enough.
I'm adding some logging to troubleshoot in the next release. It's not a fix yet, but it should provide me with some information.
Shouldn't the "Update Hubs" toggle be disabled for users in the Beta firmware channel? Almost touched it during latest f/w update, and got shudders down my spine wondering what mischief that might've caused.
I'm not sure what difference it would make; each hub will grab the latest version available to it if not already there. (That is, you won't get beta on a non-beta hub.)
I have never had a reason to reboot with the rebuild database option. Would it be something to try to see if it would make the one click button show up?
I've added some extra logging to 2.4.3.158. So the next time you're updating from 2.4.3.158 to the next build, and it doesn't show the toggle, please ping me with your hub id. Hopefully there will be something useful in the logs.
Used the update hubs toggle this morning. It updated the non-beta hubs to the the latest "official" release and the beta hub to the latest beta platform.
That's actually quite nice!
Yes it was. Made it extremely simple and much quicker. I always try and get up early on the days I am going to update so when the hubs are starting up there isn't family up going through the house. Give the hubs time to settle in. I know it probably isn't required just something I do. This new feature cuts this time down considerably.
Still don’t see the checkbox. I’ve sent you a message with my hubs ID.