My Xperia IV has been sitting in a drawer for over two years now. I’ve long since given up hope that work on SFOS will continue. After the split with Rostelcom, only the managers and the name remain.
There are actually viable alternatives that are actively being worked on. I have all of them running on my phone: Ubuntu Touch, Volla, Droidian, PostmarketOS.
I currently have nine mobile phones with alternative operating systems.
I’m keeping the Xperia IV with SFOS to post on occasionally.