I used to play a lot of older MTG formats (Legacy, Vintage, Pauper, etc)

I went looking earlier today at some of the old forums. Depending on the topic there's no posts for months or even more than a year. For example MTG Salvation has no posts on "Established" Legacy combo decks for over a year and no posts on any "Established" Legacy deck at all for over a month.

The main Vintage forum ("The Mana Drain") seems to be offline at present.

So I think much of the older player-base has probably been put off by this or that thing that Wizards has done and moved on.

That said, for probably the past decade Wizards (makers of the game) has been pivoting more towards casual games with many players and away from competitive, one-on-one duels. Their support for the Pro Tour has supposedly been declining for some time and their special products have been focused on casual group play for years (eg Nemesis and Commander). I think Magic is now built/marketed as an alternative to Dungeons and Dragons with a more fleshed-out structure (due to the cards).

I was interested in Magic as a sort of alternative to chess (to which I have returned as my main hobby), and I think that phase of Magic is mostly gone.

If you liked Magic for the strategy, I recommend lichess.org (for chess) and backgammongalaxy.com (for backgammon)

I also like to spectate Starcraft, especially Brood War. I know you mentioned your interest in SC in another post. If you don't already know, top-level Brood War tournaments are still ongoing in Korea, with the SOOP StarLeage (previously Afreeca StarLeague) just finishing a round of qualifiers and about to start a new season. A few years ago Blizzard released a "remaster" of SC1 which keeps all the same mechanics but upgrades the graphics (and fixes a few bugs, the most impactful of which is increasing particle limit so Terran Valkyries can actually shoot).

I've been following a new RTS called Battle Aces which just finished a round of closed beta. It's like Starcraft 2 minus all the homework (eg inject larva every 40 seconds). The major skills seem to be unit choice, strategy (harass, push, defend, expand, tech) and fine unit control ("micro"). Basically it looks like the Starcraft 2 I always wanted to play.

I don't think there is an active arena shooter now that Quake Champions has wrapped up its life-cycle, but I think there's surprisingly good strategic depth in that genre. Rapha's VOD reviews are really great if you've never seen them. Rapha is the Quake GOAT specifically because of his strategic understanding of the game (his aim now is about as good as other pros, but historically was worse). Watching him explain the logic behind his gameplay is quite satisfying.