Creating an issue to capture this discussion on the Kopia Discourse forum with @jkowalski. Per his reply:

"It’s not intentional, but Kopia indeed expects this to be a file and not symlink today. Happy to review PR to fix it."

What follows is an embedding of the original post I wrote up for convenience:

Behavior

Suppose I have a .kopiaignore file located in the root of my home directory, ~/.kopiaignore.

In the service of keeping the exemplified use case simple, let's have the contents read:

.cache

If I then do:

kopia snapshot estimate $HOME

... we verify that the ~/.cache dir was indeed excluded! In other words, this works as intended.

Now, let's suppose instead I have my "canonical" .kopiaignore located at ~/dotfiles/kopia/.kopiaignore and I symlink that to my home directory, such that it looks like:

# ls -la ~
lrwxrwxrwx 1 user user 27 June 12 12:17 .kopiaignore -> dotfiles/kopia/.kopiaignore

If I again do:

kopia snapshot estimate $HOME

... we observe that none of the exclusions/ignores take effect!

Commentary

While obviously the above is a bit of a contrived (simple) example, it definitely caught me off-guard in terms of a real world use case just now:

Notice how, with the top snapshot, the number of files is roughly ~178K. Then, after making the above change, swapping a static file to a symlink, all of my exclusions were ignored for the subsequent snapshot (and we see a jump up of almost ~600K files and 17GB).

Use Case

I have a dotfiles repository/dir (in the Git sense) that lives within my home directory, ~/dotfiles. This directory is synced between machines that I use and/or work on regularly.

I use GNU Stow to "install" (symlink) these dotfiles onto machines.

Example:

pwd
# ~/dotfiles

ls -l
# alacritty
# git
# gnupg
# ...
# kopia
# ...

stow kopia
cd

ls -l
# .kopiaignore -> dotfiles/kopia/.kopiaignore

TL;DR: This symlinking approach allows me to keep my Kopia exclusions in-sync between machines without having to update/overwrite N different static .kopiaignore files.

For what it's worth, I use this pattern to great success with nearly all of my "daily driver" binaries!