Affected receivers other than Slack which support templated URLs:

• Slack fix: alertmanager slack templating bug #8299, fix: correctly assign IconURL, ImageURL, ThumbURL in Slack config #8331
• Webhook pkg/alertmanager: add URL validation for Webhook receiver #8229 => fix: implement template URL validations #8307 fix: relax URL validation on WebhookConfig #8317
• PagerDuty (clientURL and image + link hrefs) Feat: Add validations for PagerDuty Receiver in Alertmanager Config CR #8113 => fix: allow templated URLs in PagerDuty config fields #8319
• Pushover (url field) Feat: Add Validations to Pushover Receiver in Alertmanager Config CR #8239 => fix: relax validation on Pushover URL #8316
• Rocket Chat (icon URL + action URLs but the main url field is fine) feat: add support for rocketchat reciever in AlertManagerConfig CRD #7328 => fix: relax validation on RocketChat template URLs #8318
• SNS Feat: Add validations to SNS Receiver in Alertmanager Config CR #8217 (this one didn't make it to v0.88.0) => fix: sns templating issue #8320

And this is not accounting for the additional validation/sanitization happening on the generated configuration...

Originally posted by @simonpasquier in #8299 (comment)