TL;DR: High quality storage will no longer be unlimited, but this won’t happen for a while; free storage remains a generous 15GB (more than enough for most people); and 100GB of storage costs two bucks a month.
Beginning June 1, any new photo or video uploaded in High quality in Google Photos will count toward your free 15 GB storage quota or any additional storage you’ve purchased as a Google One member. To make this transition easier, we’ll exempt all High quality photos and videos you back up before June 1. This includes all of the High quality photos and videos you currently store with Google Photos. Most people who back up in High quality should have years before they need to take action—in fact, we estimate that 80 percent of you should have at least three years before you reach 15 GB. You can learn more about this change in our Google Photos post.