Are long-form blog posts still relevant in the era of short attention spans?

Yes, if you manage to grab attention from the first sentences.

I think it really depends on the content of the post. Long-form blog posts can be great SEO drivers, and if you have the right audience at your disposal who will find real value in your content, then I do believe they can still find relevance in the era of short attention spans. Short-form content does seem to be dominating the market right now, though, so I would advise you to be mindful about how much time, money, and energy you're putting into your blog posts. Are you getting conversions from them? What's the cost / value balance? Analyzing metrics will be your best friend here.