I don't know what issue the uservalidationbyadmin plugin has on recent versions of Elgg. It was released for Elgg 1.8 originally, so it might require some update to work on newer versions indeed.

I guess it depends on the number of users you want to allow on your community if usage of any additional plugin makes sense. If the number is small I would say it's easier to turn off registration and just create the accounts as admin manually when needed. Usage of uservalidationbyadmin might not save time anyway as you would still have to activate the accounts manually. And you would have to deal with other people / spam bots trying to create accounts that you would have to remove (even when they are not active until admin validation).

Usage of the site access plugin (https://elgg.org/plugins/384611) might be another option. This plugin allows to restrict account created by invite code. Though I can't say how it exactly works as I've not tested it myself and can only refer to it based on the plugin description.

Iionly, thanks for the comment. I think I will turn off and manually create the users.