Probably related to https://status.heroku.com/incidents/2531 ("On April 6, 2023, between 21:20 UTC and 22:30 UTC, our Common Runtime customers were unable to provision dynos in the US and EU regions. This issue impacted starting/scaling dynos in existing apps, one-off dynos, builds, and release phases.")
Note that Slack has only sent the email about account access since 2018:
>Additional security features: As of January 2018, we began sending an email every time your account is accessed from a new device; this is a simpler and more immediate way for you to be aware of new logins to your account than periodically reviewing your access logs.
(Quote from the "Slack password reset" email they recently sent out to affected users.)