A private key leaking is bad, since anyone with the private key can decrypt data that was encrypted with it.

Traditionally, the way that leaked certs were handled was via Certificate Revocation Lists (CRL). CRLs contain lists of revoked certificates - their serial number, revocation date, and the reason why they were revoked.

However, CRLs are imperfect. Checking for revoked certificates every time you go to a site would slow things down a lot, as the lists are now too large to check and download real-time. Modern browsers and other TLS clients periodically download the lists in the background. Also, it might take a while between when the certificate is compromised and when the company notices the compromise.

Because of this, the CA/Browser forum (a group with all the major browser and TLS certificate vendors) have started dropping the max lifetime of certificates. The idea is that even if a private key does leak, the time frame that it’s usable for will be significantly shorter and any leaks should (in theory) cause less damage.

• The original maximum duration was 39 months: Three years plus an extra three months leeway for obtaining and deploying new certificates.
• March 2018: Reduced to 825 days
• September 2020: Reduced to 398 days
• March 2026: Reduced to 200 days
• March 2027: Planned to reduce to 100 days
• March 2028: Planned to reduce to 47 days

All modern deployments, regardless of if they’re using free or paid certs, should have their renewals fully-automated, so in theory the validity period shouldn’t matter as much as it did in the past. All major vendors (Let’s Encrypt, DigiCert, Sectigo, GlobalSign, AWS, SSL .com, etc) support ACME now.