i dont have any way to confirm this without an example PDF,
which triggers that dialog - i also could not locate the code
which triggers it, neither in okular nor poppler

could you try the new okular v20.12.3 to see if it behaves the
same?

could you point to an example PDF, which triggers that dialog?

it would be interesting to see how other PDF readers
(such as evince) handle thar same file

The PDF is here:

http://www.forms.ssb.gov.on.ca/mbs/ssb/forms/ssbforms.nsf/GetFileAttach/9955E~7/$File/9955E_2.pdf

Evince has show the almost empty page with the warning that it requires Adobe, which is probably a built in page, and same page is shown in Okular after that dialog warning.

This issue needs to be investigated more as it is still happening on v22.04.0
I've downloaded a random form from https://www.forms.ssb.gov.on.ca, opened it up and I get the dialog box saying it needs a newer version of Adobe Acrobat Reader DC.

When I click OK on the message, I get another dialog saying "This document has XFA forms, which are currently unsupported"
Looking up what XFA forms are, in short, it's a proprietary XML specification from Adobe: https://en.wikipedia.org/wiki/XFA

What would be the best way to go about fixing this? Do we get rid of the dialog box and references to Adobe? or maybe explain to the users in that dialog box that the pdf only supports non-free software? (and what do we suggest to the users?)
They are state forms as well, a perfect case to ask them to use open document formats. But unfortunately won't fix the Adobe warning from Okular.

i think the line can be drawn between messages which merely
indicate that something is non-free, as distinct from messages
which suggest that something non-free is "needed"

the problem i can foresee, is that it may support many other
non-free formats, which would all need new messages

isnt this just yet another document viewer? like evince, atril,
etc

evince and atril do not need patching (at least no one ever
complained about them) - parabola is already patching 'epdfview'
in this way - unless there is a compelling reason why okular
does something unique, i would not spend much time on a
replacement

at first look, the only special feature is the
"copy-protection anti-feature", which parabola disables -
if there is nothing else special about it, it could be discarded,
with no loss of functionality to the parabola system

probably all of these can read PDF files:

$ pacman -Ss document view
libre/epdfview 0.1.8-10.par1
  Lightweight PDF document viewer, without nonfree suggestions
libre/okular 20.12.3-1.parabola1 (kde-applications kde-graphics)
  Document Viewer, with copy-protection anti-feature disabled, without nonfree recommendations
extra/evince 42.2-1 (gnome)
  Document viewer (PDF, PostScript, XPS, djvu, dvi, tiff, cbr, cbz, cb7, cbt)
extra/gv 3.7.4-5
  A program to view PostScript and PDF documents
community/atril 1.26.0-1 (mate-extra)
  MATE document viewer
community/xpdf 4.04-1
  Viewer for Portable Document Format (PDF) files
community/xreader 3.3.0-1 (x-apps)
  Document viewer for files like PDF and Postscript. X-Apps Project.
community/zathura 0.4.9-1
  Minimalistic document viewer

it would be interesting to find out what each of those do,
when presented with a document in some non-free format