Well, I finally got around to upgrade my abandoned Opigno test site the other day. While I got there in the end, I have to say I had a pretty miserable time at it with navigating confusing documentation, composer dependency issues, and multiple borked upgrades where I had to start from scratch after restoring the site from backup. (To be fair though, I've heard similar stories from many people about upgrading Drupal sites from 9 to 10, so I won't blame Opigno for all of this.)
In any case, this whole debacle - and the Opigno team's reaction to it - has dissuaded me from trying to use Opigno in any of my projects. I guess my next step is to try to install Drupal LMS (which I saw was mentioned above) and see if I can migrate my test content to it.
This issue appears to have been solved in the latest version of Private Message (3.0.7) that was released yesterday.
The latest version of Simple Google Maps is compatible with Drupal 10, so I guess this issue can be closed as 'works as designed'.
I've uploaded what I've done so far to https://gitlab.com/reswild/opigno-lms-d10/ but there's still some work to be done before it installs cleanly.
If you just want a soft fork, this might not be all that much work, as you can use the existing Opigno modules either as-is, or with just some minor patching. I did some testing this weekend with updating Opigno 3.1.0 to run on Drupal 10, and while it got a bit tedious sorting through compatibility issues and other minor errors, I haven't run into any major problems so far.
My main issue when testing Opigno has been that it is too difficult to customize, so if someone wants to make a fork that is a bit more modular and customizable, I might be up for helping out with this.
This appears to be a duplicate of #3037306: Upload file is required. →
The patch in that issue worked for me.