In an announcement made on the Blizzard site, the news struck that the 'Recruit-A-Friend' system is "heading into the workshop." The team are looking at making something new and in doing so are putting the current system on hold and removing it from service, it will stop working from the 11th June 2019 but from now until then you can continue to utilise the system by linking a verified World of Warcraft account to your own, they will then have 90 days to purchase any gametime in order for you to trigger those rewards. You can see the current reward page here.
Blizzard are looking at developing a new system that makes it even more rewarding to introduce people to World of Warcraft. The population after large releases does tend to spike but then gradually decreases over time which isn't anything new to the world of gaming, but because Blizzard depend a lot on the continuing subscription revenue to supplement any further development of the gain and to cover operating costs they feel this is a program that is in a big need of an update.
We'll post more news about the new program when we hear about it.