How could this happen? We're storing deploy credentials in GitHub, and configure fully automated deploy pipelines using YAML and Shell. Escaping 101. xD
It's not clear if Mediawiki are self-hosting gitlab. If they're relying on Gitlab (the company) then I'd be worried about Gitlab running out of money and going out of business. I had a bit of an interaction with one of their sales people a while back and they appeared desperate. If self-hosting, there would be a bit of uncertainty while development reorganizes but I don't expect any long term problem.