What do people use to start a blog nowadays?

If you want a platform with ActivityPub integration (i.e. it's part of the Fediverse), then there are several options:

• https://writefreely.org/
• https://write.as/new/blog/federated
• https://joinplu.me/

There's also WordPress, which has a free hosted tier, and can alternatively be set up on your own server. There is an ActivityPub plugin for WordPress, which can be used freely on your own server, or with the paid business plan on wordpress.com.

Another possibility would be to use a Mastodon instance that doesn't limit your character count to 500, such as Aethy (which IIRC has a limit of 1M characters per post).

If I were to start a blog again (and I have considered it), then I would probably use WriteFreely, or alternatively Aethy.

Pillowfort and Dreamwidth come to mind. Pillowfort designed to be based on original Tumblr. Dreamwidth based on LiveJournal.

It feels like there should be an ActivityPub-enabled easy-to-deploy personal site setup, but I haven't been able to find one.

Neocities

You can host your blog on GitHub pages, some technical know-how required. But it's free of charge, and you have complete control over what's hosted, so you can host a plain HTML page if that's what you want.

There are quite a few static site generators which you can use. Jekyll and Hugo are the most popular, and have quite a few themes to pick from.

Tumblr?

What's up with Medium and Substack? I'm out of the loop here

I did some really basic searching and it looks like something like Yunohost might have some ActivityPub modules, and it does have some blogging modules that might work. I have not used this, so I can't say how good or bad it is, but it seemed to have potential.