I’m unsure if this is explicit/intentional behavior, so I’ll file this under maybe a bug? 
I wasn’t able to find anything in the documentation or on the forum in any case!
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!
Which brings me to: is the behavior described above intentional?
If the answer is “no”, I’d be happy to attempt to submit a pull request and/or wait patiently for this to be addressed!
If the answer is “yes”, I’d be interested to learn more!