Serious question: are they? I know they've made a lot of acquisitions over the years, but I don't recall any widely-circulated stories of "Salesforce takes beloved thing and destroys it". The major open-source-friendly company I know of that Salesforce bought was Heroku, and a lot of people seem to use Heroku without even knowing that they are owned by Salesforce (and have been since 2010). It seems like it's too early to say anything definitive about what their purchase of Slack is going to change, although Slack has been making itself quite enterprise-focused all on its own.