If you attend, leave your phone home (atypical usage), go with other people / meet them there / other people you know are there (facial recognition / gait analysis / clothing preference) those are all good data points to predict with high probability where you are and what you’re up to, especially given your typical movements, data usage patterns, purchasing habits, friends / acquaintances / social media interactions are all in at least a few databases.
Take the security measures you’re willing to make the tradeoffs for.
If history is anything to go by, we’re only ever an election, or other political churn, away from your particular sets of beliefs / identifiers being persecuted, or at least your least favourite political prisoners being released and coming after you.
And, as you allude to, relying on the security practices of others has its own problems. Even Perfect Forward Secrecy etc etc provides little help against Rubber Hose Cryptography.