I'm a new player and this may be a dumb question, but why are some servants like Jeanne d'Arc Alter and Scáthach are not summonable? Are they going to be in future summoning events, if so which ones?
These are Event-Limited Servants, like most of high-rarity Servants in the game. Almost every event has it's own pool of limited Servants and you have a chance to get them only when special Summoning Campaign is active. Jeanne Alter was available to summon only through Da Vinci's event, so we have to wait until event's re-run.
Basically it means they're only available during specific summoning campaigns. A few other examples would be Gilgamesh and Nero Bride. You literally can't summon them unless they're specifically added to the summoning pool by the developers and it's always a temporary thing. As for when they'll be in the pool next, well some of them won't be, and some of them will. For example Brynhild has so far had only one summoning campaign to my knowledge, even on the JP Server, so the only way you'll get her is if they add her to one of the guaranteed SSR summonings which they occasionally do. Other servants, like Gil and Scathach, are super popular so they add them any chance they get so they can rake in the whalers money. If you want to see an upcoming list of events just go to the FGO wiki and look at the list for the JP servers events. We have the exact same events in the same order as them, we just have slightly different names in some cases and the time frame for when they occur is moved up.
