This does not seem to work in Odoo 15 (on odoo.sh). Still digging, this seems to have been a common request dating back 8 years, but this is the newest thread. I will update if I figure out what does work.

I was incorrect, the emails were disabled. Just coming from Outlook sync rather than Odoo.

do you mean block calendar emails stop reminder before meeting or hide button email in calender event , give us more details please to able help you

it does work, the issue is that you have to write True not true, let me provide the piece of code where odoo manage this system parameter: https://postimg.cc/MnCsz54y (or search def _send_mail_to_attendees)