how has writing at Steemit (or other places) affected your English skills? Has actively writing improved your abilities even though that was not the goal, and has it affected your spoken levels?

It certainly improves the more you write but it improves more when you read from a person who writes better than you. So if you try to write better you will get better, and if you talk with people better in writing
than you and you get used to their 'style' will help you improve.

I don't practice my conversation skills so I'm bad at spoken English, but I'm pretty sure if you improve your writing some of the improvement will get into your speaking and vice versa...

That's how is it for me... Hope it helped.