You can read their announcement of it here. This is because they are in the process of digitizing all their microfilm for online access and estimate they will be complete by the end of 2020. Of course, I hugely appreciate the massive undertaking of scanning so many records from microfilm and making them available for free, the contribution their organization has made to the genealogy community is outstanding, but it is still disappointing to know there is going to be an almost 3 year downtime in which many records won't be accessible at all. That's assuming their projection is correct and they complete everything by the end of 2020. Hopefully, it does not take longer than expected.
According to FamilySearch, they have 2.4 million microfilms at the Family History Library in Salt Lake City, and they have so far digitized 1.5 million of those. That means there is still almost a million more films (37.5%) which need to be scanned. Granted, they have prioritized those which are most frequently ordered and already digitized them, but there are probably thousands (millions?) of people out there who need access to those roughly 900,000 films still not scanned, who will no longer have it for the next 2-3 years. I am one of them, in the last 2 years I have order dozens of microfilm which not only was not digitized at the time, but remains still un-scanned today.
I am just surprised that they aren't waiting until everything, or at least close to everything is done before discontinuing it. They've only just passed the half way point of completion. I have seen arguments as to why they are discontinuing the microfilm service, and they all make sense for discontinuing it eventually, just not right now, before their scanning project is at least a little closer to being complete. It is certainly disappointing, but it's not like it's the first time I've been forced to wait years for access to certain records. Fingers crossed their timeline projection is accurate.