[2021] New player here: How "dead" is this game (i.e. Team Fortress 2. Little development, but the community thrives) and is it casual-friendly? Pay-to-Win?

Depends on your platform. Xbox has been around a while so it's a smaller and more stable crowd. PS4 is just about hitting the end of its honeymoon phase so the hype crowd will start thinning out so it will feel a little more empty but will still have a healthy playerbase, just less crowded. PC is going to be jack-packed for probably about 6 months before it stabilizes into a longer term crowd.

It's grindy, and people who are failing at progression complain its P2W but it absolutely isn't. I've been playing for 11 months and I've spent $12 total and I'm endgame. You can pay real money for items that make your grind a little easier, and some people sell those items to the player market so you can buy those things for ingame currency as well.

It's under constant development. Maybe a little too focused on adding new things and not enough on polishing existing content. There is some content-drought at the end so mechanics are in place to keep you from maxing out too quickly. For many, endgame is pvp. For me, enddgame is my alts

Right, so you CAN win without paying. You can spend money to get ahead faster, but it isn't required or necessary and that makes all the difference. No matter how much money you pour in, you still have to grind like everybody else to get anywhere that matters