As far as old messages goes, Matrix rooms do not allow new participants to see old history, unless explicitly enabled.
>if you have 250 people in the chat I'm not sure it'll be very effective. It only takes one of those people to leak the messages.
Ultimately, the system is only good when the members of a room deliberately leaking messages is the primary concern; It means the system is working as intended, thus privacy is a matter of trust on the conversation members, as it is away from keyboard.
I can't suggest XMPP to non-technical people, because I know they'll end up talking to each other with neither e2ee nor awareness of lack of e2ee.
and how is it achieved when you want that? Standard feature that is expected by people using Telegram, Matrix, Slack, etc.
When you get into groups with hundreds of members I think Slack and Discord are probably the dominant chat apps there.