This was the same in Android 5 as in Android 8 (and hence, I assume, Android 7): as said above, you can only uninstall updates to pre-installed system apps.
The reason is that for a system app there is a copy which is part of the ROM. Updates are however installed to the same space as user apps. So if you remove the update, the original copy (in the ROM) is still present. But if you get a second update to it that replaces the first update, so uninstalling updates always takes you back to the original version rather than the previous update.
For a user-installed app there is no other copy in the ROM. When you update one of these apps the update replaces the previous version, so removing the update would remove your only copy of the app. That's why for user-installed apps there is no "uninstall updates" option, because uninstalling the update uninstalls the only copy of the app you have.
If you want to roll a user-installed app back to a previous version you need to install a copy of the older version of the app. Be careful where you get it from - you don't want to install a copy from some random site and find out that someone added malware to it! apkmirror.com is a safe site, and older versions of many apps can be found there (not all: not all developers want their apps to be archived this way, and obviously you won't find paid apps there). Or if you have a problem with an update you could contact the developer: some will send you an older version if the current one doesn't work for you.