Related to this, there is a note in the release notes for upgrading to Debian 13: https://www.debian.org/releases/trixie/release-notes/issues.html#ensure-boot-has-enough-free-space

It mentions an increase in the minimum requirements for /boot partition.

With the size of modern linux kernels, I think 1GiB for a /boot partition is the absolute minimum I would go for a current full-sized distributuon. You'll run into these out-of-space issues on updates all the time otherwise.

Checking the /boot size on my Fedora install, I partitioned out a gibibyte for the 3 kernel plus recovery kernel setup, which takes up about 338 MiB in total. Depending on out-of-tree kernel modules and bootloader modifications installed, your initramfs images could be larger. A few things to look for:

• the size of your current initramfs and vmlinuz image(s)
• any kernel modules you needed to install alongside your system (v4l2-loopback, nvidia, realtek, etc.)
• If there are other large files present in the boot partition

If everything there looks fine and/or is necessary, you might need to expand your /boot partition (either reinstall if new system or offline partition shrinking, moving after a data backup if you have personal files you care about).

I ran into a problem like this once, and after banging my head against the wall for days it turned out the solution was just running

sudo apt autoremove

That took care of removing a bunch of old stuff in the boot partition that I will not pretend to understand.

Not having a separate boot partition is the way I roll, I think. The way debian stores old kernels even 1gb wouldn't be much.