At this stage it could go either way. As has been noted, the 5 lost support at iOS11 which would suggest the 5s will loose support at iOS12. However, the s models have traditionally had an extra year of support - the 3Gs got iOS6, the 4S got iOS9 so it wouldn't suprise me if the 4S gets iOS12, especially if iOS 12 is more of of a performance/bugfix update.