https://json.blog/2021/04/26/on-politics-at.html
To some folks, "politics at work" means "endless battle royale political debates among coworkers in a Slack all day long; why wouldn't you be against that?
But to others, "politics at work" means "are we paying women and men the same for doing the same job?" and "are our recruiting practices leading to a non-diverse workforce and missing out on great people who should work here?" and "what problems does our company sole, and for whom?". These are really important questions. Why wouldn't you want to bring them up at work?
I'm giving Basecamp the benefit of the doubt and assuming they're trying to stave off the former. But it's not that easy to do that without also affecting the latter, because if you ask questions that boil down to is our company discriminating, even unintentionally and do our LGBT and minority employees feel as safe and valued as our straight cis white employees, someone will be super upset that you're "bringing politics into the workplace." (I guarantee someone reading this is thinking "ugh, 'cis' is a slur, why you gotta be so political," and, bang: somebody has just made recognizing the existence of trans people into something inherently political, and now we get to argue over whether that somebody is me or the cis-is-a-slur guy.)
Here's a thought: if your goal is to try to keep company Slack channels civil and focused on work, then don't say "don't be political." Say "keep company Slack channels civil and focused on work."