Hi Francesco
Let me step in here with some comments, please let me know your thoughts at the end.
First, the older ticket you linked to was about inserting Views with search filters on archive pages (possibly via a widget), whereas this thread if I'm understanding correctly refers to a custom archive with search filters, and that's what I'm addressing here.
If you have a taxonomy filter for, say, a colour taxonomy, on the post type archive the filter will include all available terms.
But if you use the same custom archive assigned to the colour taxonomy archive, when you visit the, say, archive for the red term, then the filter will only include the red term, the other colour terms are not included.
That is by design, because a custom archive with search works by adding filters to the main query, and on the archive for the term red that term is already specified in the main query, and you can't filter for blue posts on the red archive where the red term has already been set.
I think you understand this, but I'm repeating it for clarity.
Now, you say that you would want the colour filter to include all the terms, not just red, on the red archive.
The problem with your suggestion for modifying the plugin code is that it won't work.
Try it yourself.
I didn't add a filter action, I just used false for the purposes of testing:
if ( $this->in_this_tax_archive_page && false ) {
It does work inasmuch as all of the terms are then included in the taxonomy filter dropdown—but they don't work.
Choosing blue, for example, has no effect, the page updates and posts with the term red (from the current archive main query) are the posts which are returned.
And it is never going to work in the sense of trying to fight the main query.
What we've suggested in the past is that users replace the taxonomy "filter" on a custom archive for the same taxonomy with a list of links in a dropdown. Persisting with the colour example, on the red archive the dropdown would show red, blue, and green, but red would be inert (effectively already selected), while blue and green wouldn't filter the existing page but instead would be links to the blue archive and to the green archive respectively.
The drawback with such an approach is that if the user had already selected some other filters (e.g. custom field filters), then selected the taxonomy filter which navigated to another archive URL, the previously selected filter values would be lost, they would need to enter them again.
