If my child were going to a sleepover and then came home and said they had been allowed to keep their phones all night, and that people had been on social media (and I know Ops DD has a brick phone not smart phone) and sending pictures and all kinds of messages, I would hold the parents responsible for allowing them to have those phones overnight, when all of the recommendations are not to let groups have phones overnight, because these exact things happen.
I would rather my child was asked to leave their phone downstairs and knew they could speak to the host to make contact with home if it became necessary, than there was the risk of this behaviour - which is fairly likely if a group of kids have phones in their rooms over night.
In the end, a lot of this is about assessing risk accurately. The risk of desperately needing to contact a parent and only your own mobile being a possible way to do that, is very very small. Most of the awful things that could happen are extremely unlikely and would not be prevented by having a mobile. Deciding this risk is high at a sleepover should result in not sending a child to the sleepover, because that is the action to keep them safe, not sending them with a phone.
The risk of a group of kids using phones to access social media, the Internet, to take photos or to send inappropriate messages is very high. These things in themselves can be very harmful. Not having phones or other devices in bedrooms would significantly reduce this risk.
So you weigh one against the other. You know that kids without mobiles in bedrooms are not being denied access to an adult or even the opportunity to speak with a parent if they need to (there are landlines and if there were a need, the host would give access to the mobile for use) so if contact is needed, it is possible, but the risks of inappropriate mobile use are severely reduced.
What is this argument about personal property and a right to supersede all house rules? A child could have a phone, or a tablet with porn on it, or alcohol, or a weapon...or any number of items which are undesirable or could be used in an undesirable way. The fact they are owned by the child does not give them a right to keep them with them through the visit. A child going to a sleepover has to come under the jurisdiction of the adult in charge - yes there are ways and means to explain your house rules and to make them happen, but a host must have the right to do that, and if that means asking a child not to have a phone or anything else in the bedroom, then so be it. I have known lots of hosts ask children not to have the £8 of sweets they bought earlier in the day in the bedroom - because they have seen children very sick in the night before following having them - similar thing.
I wouldn't be willing to have a child sleepover at my house if either they or their parents weren't prepared to let me make decisions about their well being. As the responsible adult, I have to be allowed to make decisions to keep them safe. Children's choices are sometimes sensible and appropriate and sometimes they are not - by nature of age and maturity they need adult supervision and a host would be negligent not to provide it.
And yes, some children might have become quite dependent on their phone and feel reassured by having it by their bed at night. I would be happy for them to phone home before bed and/first thing in the morning if they wanted to, but not to allow that phone and all the others because that puts everyone at risk. If they didn't want to stay without their phone nearby, they would simply need to go home.
Hosts will have all kinds of house rules that might differ to your own. They cannot and cannot be expected to list every possible house rule which might come up to the parents of a sleepover child - it would impractical and daft. Parents sending their children don't and cannot expect everything to be laid out and if they have special requirements should discuss how the host feels about them in advance. Sending your child to a sleepover is agreeing to their rules and accepting they take responsibility for that short while.
The bottom line, is that if parents are not prepared to let the host make decisions as they see fit whilst their child is in their care, they shouldn't send their child to a sleepover there.