On latest OsClass if you deactivate a listing you can still view it via URL and without any notification that listing is deactivated. It’s ok to be seen via URL so it still shows on Google results but it should have a notification that item is deactivated, like it’s for items marked as sold.
Moreover phone and message should be disabled for deactivated listings.

Correct, so user that created listing without login can see this listing.
Listing is not in search and is not included anywhere on site, so cannot be found by visitors.

Correct, so user that created listing without login can see this listing.
Listing is not in search and is not included anywhere on site, so cannot be found by visitors.

User created that listing being logged in. Anyone can see it, it can be found on google by customers and they will send message and/or call seller to ask about the item, you should definitely add notification that listing is deactivated.

Noted.

Noted.

Any ETA to fix this?

Checked, inactive listing is 404 for user that does not own listing or user that is not logged as admin.
You can test:
https://dev7.abprofitrade.eu/en/for-sale/dvd/cds-records/delta-8-thc-disposable-vape-variety-4-pack-bundle-i5632