I don't think this is related at all. Regarding Slack, according to crt.sh, they have a history of using DigiCert certificates on slack-edge.com. They now intend to use Let's Encrypt certificates.
On a.slack-edge.com, you can still find a DigiCert certificate, while on b.slack-edge.com, there's currently the Let's Encrypt long chain deployed. That's presumably what they're going to roll out everwhere.
(Also previously noted here Email from Slack regarding ISRG Root X1 - #3 by mcpherrinm)