Apparently this only applies to the Mix/Master subscription, so it's 'optional' for those who wish to go that route...
"Now, if you own Slate Digital plugins, do not fear. We're not going to penalize you or stop supporting you, and you can still purchase any new plugins you want as you normally do, and you'll be completely unaffected. We are not getting rid of perpetual licenses. But for Slate Digital customers who want access to more amazing tools on their terms and at a price that they can easily justify, the Mix/Master or the Mix/Master/FX bundle is for you. And to help you get started, if you sign up for the subscription within 30 days of its release, you'll get credited for free months depending on the amount of Slate Digital plugins you have!"
"With the Mix/Master Subscription, you can use your plugins
when you want. And if you go away for a bit or take some time off from the studio,
pause your subscription and save some money. There's no penalty, and you have complete freedom. Reactivating is easy."
"And with the ilok you can take your subscription anywhere.. you can use your Mix/Master bundle at any studio in the world."