"Reinstalling" Ventura over High Sierra -- "No Data Loss"???

Where did you install Ventura? If you happened to select the "Data" volume or some other volume, then the Ventura installation would not have your old macOS user accounts. Look in the "/Users/Shared" folder to see if there is a "Relocated Items" folder.

If a Relocated Items folder appears on your Mac after upgrading macOS - Apple Support

As long as you did not use Disk Utility to erase anything, then the data should still be there somewhere assuming there are no hardware issues which could have caused problems or data loss.

Run the following command in the Terminal app so we can see the drive layout:

diskutil  list  internal

BDAqua wrote:

Are we forgetting that Ventura splits the drive into 2 partitions & HS did not?

Volumes ?

Reinstalling macOS from the recovery menu doesn't erase your data. However, if there are corruptions during installation, your data may also be damaged or lost. Hopefully you have a backup to restore from. Or you need to send your Mac to the data recovery center to get data back.

OP wrote " Using CMD-R Recovery menu, I tried the "Reinstall High Sierra" option on a non-booting 2017 iMac. But this caused the "Recovery server could not be contacted" message despite being connected to wifi (which it never prompted me to do, btw)."

That is sadly true as Apple has Removed the High Sierra macOS 10.13 from the Apple Recovery Servers about a year or so ago.

OP wrote " So I used Opt-CMD-R (internet recovery), and it offered to "Reinstall Ventura".

Yup, this would expressly mean the " non-booting 2017 iMac " already had a version of Ventura already installed though not functional in the Normal Bootup Mode

A " Reinstall Ventura " is what we sometimes call " An In-Place " installation meaning installing Over the Existing and already installed version - Ventura

Often used to fix corrupted or missing elements of the Operating System

Normally, in a good computer practice, one would have been using some type of Backup Regime - incase things when sideways

If you have used or are using Time Machine Backup Utility  you maybe able to Restore the Deleted Files 

For future purposes

To truly protect your Non replaceable Data

Have a 3-2-1 Rescue Plan in place and always current

3 Backups using 2 methods and 1 off site incase of natural disaster or un-natural disaster.

Each of the above should be done to a Dedicated Single Purposed External Drive 

I use it to augment and not replace Time Machine Backup

https://bombich.com

Are we forgetting that Ventura splits the drive into 2 partitions & HS did not?

Gulp, do you have any backups like Time Machine?

Ahem... yep!:)

😎