Looks like some code issue from their side.
A ton of companies are using these tools literally 24/7, at most of my jobs Github/Gitlab being down meant most of us would just be spending half of our time faking working (rare are the people who could just do "offline" dev for half a day without any external input/output)
Slack could be down it had less impact overall.
[1] https://git-scm.com/book/en/v2/Distributed-Git-Distributed-W...
The complexity of technical systems is massive especially when they are constantly changing. It's not helped that these companies have high availability requirements and need to be online 100% of the time even when making updates.
Makes me want to do embedded programming.
Reminds me of the time when Twitter had an outage, because an FB outage made people go on Twitter to ask whether FB is down