So I've been using Stretch as a production server for a small academic research office.
Had a nice ~500 or so days of uptime, and I kept it up to date as updates rolled.
This summer, Stretch was phased out from LTS. So I updated to 10, then 11 (Bullseye). Since then, I feel like I have to reboot after a kernel update maybe, once a month ? (Rebooted twice since June when I upgraded the OS).
I am a bit annoyed at being 'forced' to reboot that often for changes to be applied. I need to find time windows and/or advise with userbase to prevent service outage. And well, it is new to me that a Linux distro needed to be rebooted that often to be keep up to date. (Also, but not that important, I think about my uptime record that need to be bested!)
There is this post here saying it is recommended to reboot after kernel upgrades Debian: Kernel update using APT - reboot required to take effect? which I understand, but it was not like that before, isn't?
Is there a new paradigm for Debian where reboots were not mandatory before Bullseye but now are?
I don't get what is your concern. Yes, to update kernel you need to reboot. It was always like this, 25 years ago, 20 years ago, 10 years ago, 5 years ago, now. There were attempts to implement live kernel patching to avoid this reboot in other Linux distros, but this is not widespread and Debian never supported any of it.
Debian doesn't require you to reboot. It will continue to run on old kernel, until you actually reboot. Again, it was always like this. Maybe they now saying it more clear and more loud, I don't know. I don't feel something changed.
If you want to be able to have a service available without interruption, you have to implement a cluster (multiple servers) with rolling update. Then you'll be able to reboot/upgrade/whatever your systems one by one. Single server solution could not possibly be highly available anyway.
And remember, big uptime is like big penis: probably looks nice (for geeks), but doesn't mean you're superior. Absolutely useless metric. Better concentrate on something more valuable, like customer satisfaction.