A sadly-now-deleted comment mentioned certdays.sh, which a quick search shows was posted by its author here:

https://news.ycombinator.com/item?id=24967320 (Nov 2020)

Edit: ok, what the hell:

addons.thunderbird.net SSL certificate has expired - https://news.ycombinator.com/item?id=28116117 - Aug 2021 (55 comments)

W3.org Cert Expired - https://news.ycombinator.com/item?id=27363813 - June 2021 (119 comments)

Cmake.org SSL server certificate have expired - https://news.ycombinator.com/item?id=27314666 - May 2021 (3 comments)

.NET NuGet Package Validation Broken: Certificate Expired - https://news.ycombinator.com/item?id=25929235 - Jan 2021 (4 comments)

Krebsonsecurity.com has an expired SSL certificate - https://news.ycombinator.com/item?id=25132182 - Nov 2020 (63 comments)

Intermediate certificate used for issuance of Comodo certs has expired - https://news.ycombinator.com/item?id=23360624 - May 2020 (6 comments)

Techcrunch SSL Cert Expired - https://news.ycombinator.com/item?id=22422227 - Feb 2020 (95 comments)

Microsoft Teams outage due to expired certificate - https://news.ycombinator.com/item?id=22227266 - Feb 2020 (172 comments)

Mozilla Expired Add-Ons Certificate Post-Mortem - https://news.ycombinator.com/item?id=20423221 - July 2019 (3 comments)

DNSCrypt – how expired certificates became a thing of the past - https://news.ycombinator.com/item?id=19830910 - May 2019 (13 comments)

Apidock.com SSL cert is expired - https://news.ycombinator.com/item?id=19731409 - April 2019 (3 comments)

O2 outage due to expired Ericsson certificate - https://news.ycombinator.com/item?id=18622169 - Dec 2018 (89 comments)

Over half the firmwares uploaded to TCSL Armor have invalid certificates - https://news.ycombinator.com/item?id=17993511 - Sept 2018 (17 comments)

All of Oculus’s Rift headsets have stopped working due to an expired certificate - https://news.ycombinator.com/item?id=16541235 - March 2018 (376 comments)

Ask HN: Does YC blog has an expired SSL certificate? - https://news.ycombinator.com/item?id=14580560 - June 2017 (3 comments)

Mac Store Apps Stopped Working Due to Expired Security Certificate - https://news.ycombinator.com/item?id=10560634 - Nov 2015 (152 comments)

Manjaro Blames GlobalSign for SSL Cert expiry after not receiving “extension” - https://news.ycombinator.com/item?id=9366653 - April 2015 (10 comments)

Expired SSL certificate - https://news.ycombinator.com/item?id=9346508 - April 2015 (67 comments)

Google SSL cert just expired - https://news.ycombinator.com/item?id=9321432 - April 2015 (5 comments)

Gmail SMTP Certificate Just Expired - https://news.ycombinator.com/item?id=9321184 - April 2015 (4 comments)

index.docker.io was serving an expired certificate - https://news.ycombinator.com/item?id=9241802 - March 2015 (24 comments)

GitHub and BitBucket's SSL Provider's Cert has expired - https://news.ycombinator.com/item?id=8090394 - July 2014 (43 comments)

Expired SSL certificate. Shall I use the site anyway? - https://news.ycombinator.com/item?id=7830388 - June 2014 (3 comments)

Never mind Heartbleed, Santander are using a cert that expired a year ago - https://news.ycombinator.com/item?id=7572414 - April 2014 (6 comments)

Mailgun Down – SSL cert is expired - https://news.ycombinator.com/item?id=7417485 - March 2014 (53 comments)

Windows Azure Storage certificate expired? - https://news.ycombinator.com/item?id=5266947 - Feb 2013 (78 comments)

Over a month later and Comcast still doesn't know how to SSL - https://news.ycombinator.com/item?id=4707854 - Oct 2012 (49 comments)

Ietf.org certificate is expired - https://news.ycombinator.com/item?id=2936159 - Aug 2011 (4 comments)

Wikipedia's https certificate expired. - https://news.ycombinator.com/item?id=2376115 - March 2011 (9 comments)