How are you attempting to update your system?

I would use

# apt update
# apt upgrade
# apt full-upgrade

Or perhaps

# apt update
# aptitude safe-upgrade
# aptitude full-upgrade

Note that full upgrades are sometimes needed in testing/unstable because obsolete packages have to be removed. Also note that full upgrades can break testing/unstable systems if a transition is underway.

https://release.debian.org/transitions/

Yes, I'm updating using the first method (except the third line, which I see will remove a couple of Python packages).

# apt full-upgrade
The following packages will be REMOVED:
jack libpython-dbg libpython-dev python python-all python-all-dbg python-all-dev python-cddb python-dbg python-dev python-eyed3 python-magic python-ogg python-pathlib python-pyvorbis python2-dbg python2-dev

Not long after I posted this thread, I saw that the compiler packages mentioned up above become possible to upgrade. I upgraded without a single problem.

I've been using the "unstable" Ceres for a few weeks. It's going along just fine, perhaps better than Chimaera or even Beowulf. The only problem is I've been noticing more packages are kept back when I check for updates. I just put the repositories for Chimaera and Beowulf back on (temporarily) in order to recover some missing Python packages (due to still being at 2.7.17-2, despite Python2 and others being at 2.7.18-2). I also did that to make Youtube-dl functional again.

Now, I'm seeing a growing list of packages that can't be upgraded. In this situation, should I just ignore it and wait until newer versions are available from upstream?

The following packages have been kept back:
 cpp-10 cpp-9 g++-10 g++-10-multilib g++-9 g++-9-multilib gcc-10 gcc-10-base gcc-10-base:i386 gcc-10-multilib gcc-9 gcc-9-base gcc-9-multilib gfortran-9 lib32asan5 lib32asan6 lib32atomic1 lib32gcc-10-dev lib32gcc-9-dev lib32gcc-s1 lib32gomp1 lib32itm1 lib32quadmath0 lib32stdc++-10-dev lib32stdc++-9-dev lib32stdc++6 lib32stdc++6-10-dbg lib32ubsan1 libasan5 libasan6 libatomic1 libatomic1:i386 libcc1-0 libgcc-10-dev libgcc-9-dev libgcc-s1 libgcc-s1:i386 libgfortran-9-dev libgfortran5 libgomp1 libgomp1:i386 libitm1 liblsan0 libpython-all-dbg libpython-all-dev libpython2-dbg libpython2-dev libpython2-stdlib libquadmath0 libstdc++-10-dev libstdc++-9-dev libstdc++6 libstdc++6:i386 libtsan0 libubsan1 libx32asan5 libx32asan6 libx32atomic1 libx32gcc-10-dev libx32gcc-9-dev libx32gcc-s1 libx32gomp1 libx32itm1 libx32quadmath0 libx32stdc++-10-dev libx32stdc++-9-dev libx32stdc++6 libx32stdc++6-10-dbg libx32ubsan1 python-all python-all-dbg python-all-dev python2 python2-dbg python2-dev python2-minimal

I just apt update/upgraded my system, and these all upgraded without incident. Except 3 python2 packages - there has been some funny business with python2 for a while.

I would use

# apt update
# apt upgrade
# apt full-upgrade

Or perhaps

# apt update
# aptitude safe-upgrade
# aptitude full-upgrade

Note that full upgrades are sometimes needed in testing/unstable because obsolete packages have to be removed. Also note that full upgrades can break testing/unstable systems if a transition is underway.

https://release.debian.org/transitions/

Now, I'm seeing a growing list of packages that can't be upgraded. In this situation, should I just ignore it and wait until newer versions are available from upstream?

The following packages have been kept back:
 cpp-10 cpp-9 g++-10 g++-10-multilib g++-9 g++-9-multilib gcc-10 gcc-10-base gcc-10-base:i386 gcc-10-multilib gcc-9 gcc-9-base gcc-9-multilib gfortran-9 lib32asan5 lib32asan6 lib32atomic1 lib32gcc-10-dev lib32gcc-9-dev lib32gcc-s1 lib32gomp1 lib32itm1 lib32quadmath0 lib32stdc++-10-dev lib32stdc++-9-dev lib32stdc++6 lib32stdc++6-10-dbg lib32ubsan1 libasan5 libasan6 libatomic1 libatomic1:i386 libcc1-0 libgcc-10-dev libgcc-9-dev libgcc-s1 libgcc-s1:i386 libgfortran-9-dev libgfortran5 libgomp1 libgomp1:i386 libitm1 liblsan0 libpython-all-dbg libpython-all-dev libpython2-dbg libpython2-dev libpython2-stdlib libquadmath0 libstdc++-10-dev libstdc++-9-dev libstdc++6 libstdc++6:i386 libtsan0 libubsan1 libx32asan5 libx32asan6 libx32atomic1 libx32gcc-10-dev libx32gcc-9-dev libx32gcc-s1 libx32gomp1 libx32itm1 libx32quadmath0 libx32stdc++-10-dev libx32stdc++-9-dev libx32stdc++6 libx32stdc++6-10-dbg libx32ubsan1 python-all python-all-dbg python-all-dev python2 python2-dbg python2-dev python2-minimal