I would check out the school website to see if they have any formal communication policy.
I would then go into school and ask to see the head of safeguarding and ask what their communication policy is - in general terms and specifically for those people who need to not be on social media for safety reasons.
If they reply with a breezy ‘oh everyone uses SM’ or similar, as to see their risk assessment for only using SM for people that are at risk and can’t use SM. I’m assuming that the school will have some sort of form that they allow you to opt out of photos and ensure that you/your dcs are never in pictures where they or the school can be identified (and I’m assuming you’re signed up to this as it fits with what you’ve said) so they should be aware of the importance of these provisions.
Depending on their reaction to you, I would tell them that you’re raising this as a formal safeguarding complaint (and cc in the chair of the governors, the governor responsible for safeguarding and the safeguarding contact at the local council). Discuss with them what you need from communications from school and see what they suggest and ask themto get back to you within a week with practical implementable (and implemented!) changes to the communications with home as well as the new policy updated to reflect the new risk assessment.
Then make sure you send it all in writing and cc appropriately so it doesn’t get forgotten.
And see what happens. Safeguarding aside they shouldn’t be organising things with such short notice.
Twitter and Facebook are great for reminders, to give timely updates on trips (we’ve arrived safely! We’ve just spent an hour in a traffic jam coming home so we will be at least an hour late getting back after the trip to the castle! Congratulations to the y6 football team for winning their match today! Remember to dress up warm for fireworks night display on Saturday! Etc) but not as a main source of info.
Remember to point out how badly it reflects on the school if their website is so lacking in info on dates too. And whilst there will always be new opportunities that pop up at the last minute, it is reasonable to assume that they will do the nativity play, the Easter bonnet competition, sports day, summer feteetc as they do every year so those things should be there from the beginning of the year. Most sensible schools put in a couple of reserve dates if things aren’t completely firmed up or may change eg waiting for confirmation from the touring panto company, extra dates reserved for sports day in case it rains or is too hot...
Get other parents to also complain in writing about the lack of notice re the phonics evening.
See what happens and keep on at them every time they exclude you by giving too little notice and/or only using SM.