Will custom kernels running on 486es still be supported, or is Debian
intending to desupport the 486 in userland as well?
They might still work. gcc is now configured to target 586 and above,
but I don't think there are any 586-only instructions that it will
automatically generate.
However, given that this fatal bug has not (so far as I know) been
reported in the 5 years since it was introduced in unstable, I suspect
you're one of a very few people still using Debian on a 486, and you may
find many other things broken.