Many websites these days use the free SSL certificates from LetsEncrypt. These are fully automated, (again) free and very convenient. On the downside, if something gets changed on their end, it may affect a lot of users.
One of such changes happened with their ROOT certificate back in September 2021: DST Root CA X3 Expiration.
If you have an older iPhone or iPad running older iOS and Safari versions (I have an iPhone 5s running 9.3), you may be affected. But the good news is there's a fix.
Manually install the missing ROOT cert by following these instructions: https://community.letsencrypt.org/t/isrg-root-x1-not-supported-on-ios-9-3-5/162193/15