working:
> dig api.slack.com

; <<>> DiG 9.11.5-P4-5.1ubuntu2.1-Ubuntu <<>> api.slack.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 58061
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 65494
;; QUESTION SECTION:
;api.slack.com.                 IN      A

;; ANSWER SECTION:
api.slack.com.          59      IN      A       54.203.112.76

;; Query time: 63 msec
;; SERVER: 127.0.0.53#53(127.0.0.53)
;; WHEN: Mon Apr 27 16:54:06 MDT 2020
;; MSG SIZE  rcvd: 58

No longer working:
> dig api.slack.com

; <<>> DiG 9.11.5-P4-5.1ubuntu2.1-Ubuntu <<>> api.slack.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 3940
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 65494
;; QUESTION SECTION:
;api.slack.com.                 IN      A

;; Query time: 561 msec
;; SERVER: 127.0.0.53#53(127.0.0.53)
;; WHEN: Mon Apr 27 16:56:40 MDT 2020
;; MSG SIZE  rcvd: 42