Turbo mode was one of the headline announcements when Dota 2's Dueling Fates update hit. It's a version of the main 5v5 mode where everyone picks the hero they feel like playing (no bans or particular pick order or anything) but it's a lot shorter. It's also the best thing Valve have added to the game in years.
I think Turbo mode got a bit lost in the excitement about two new heroes—one new character is such a rare occurrence in Dota so two is like early Christmas. Plus the patch notes themselves were more than 11,000 words long, each sentence detailing a new change. With the sheer volume and complexity of stuff to parse—nap time for neutral monsters, free mangoes for particular heroes—I feel like Turbo mode ended up not exactly overlooked but, as one of the changes which was actually easy to understand, you could give it a nod and move on.
The way it achieves brevity is as follows: each hero earns gold and experience faster so they can buy items faster and level up faster; defensive towers are weaker so it's not as hard to knock them down; respawn times are reduced meaning you spend less time definitely not grumpily tabbing out while dead; and you can buy anything from anywhere instead of summoning a long-suffering donkey or dithering in a side shop.
All of this leads to a quicker, lower-stakes style of match which is more forgiving to newcomers and rusty returnees alike. I think it might actually end up being my main mode in the same way that ARAM (all random, all mid) is in League of Legends.
I think the best thing here is that pretty much everything about the game is set up to lower toxicity. The matches are shorter so if you botch things up the experience will be over soon. I usually take forever to try out a new champion or step outside my comfort zone because the idea of potentially wasting an hour is a significant obstacle. It also carries the threat of other people being furious about you wasting their time.
Defensive tower changes also mean that you don't get stuck trying to end a game. The respawn timer tweaks mean there's less downtime and thus fewer opportunities to get bored, or to be significantly absent from fights in a way that your team can feel let down by a careless death.
That doesn't mean stakes have vanished entirely. In one match I played, an Anti-Mage who had been doing sterling work needed to leave. Three of the remaining four of us were pretty chill about it and one person was hell-bent on reporting them for the abandon. I mean, I get that it's frustrating to have wasted time and we probably lost the game based on that sudden absence. But it was nice that most people on the team were typing variants of "it's fine—it's only Turbo mode" into chat instead of having a big old barney.
Dota 2 has had various forms of practice mode for a while. There's a sort of singleplayer tutorial thing where a parrot teaches a dwarf with a gun some basic principles; there are a bunch of custom game modes which let you get practice last-hitting creeps and things like that; there's a third-party thing which lets you practice the typed combos which produce the ten Invoker spells; there's a demo hero mode which lets you set your hero level, toggle invulnerability, experiment with builds and so on in an isolated mid lane scenario. None of those feel sufficiently like a real game to help ease you into playing "real" Dota matches.
Turbo mode tends to head to clown town after about 10 minutes —for me it's the point at which I've earned more gold than I would ever normally do, bought all my support items and am eyeing up a casual Mjollnir—but before it does you still get a bit of the laning experience. You still do some warding or some ambushing. You still need to figure out pathways around the jungle, or guard against being jumped by a suddenly invisible enemy team. You still need to actually get to grips with your character's abilities. It's just that you're a kazillion times less likely to get abuse for not knowing that stuff perfectly.
With destination clown town in mind it's not really a place where you'll learn late game strategies or how to break into a foe's base (although a couple of the changes in 7.07 might help with the latter anyway). But it's far more of a helpful playground for a Dota experience than the previous options, and far more likely to be populated given it's an official game mode rather than something hidden within custom games.
I stopped playing Dota 2 a fair while ago because I'd ossified into a specific type of support role and the game didn't really support breaking out of that in a fun way. Since 7.07 I've actually been playing and—perhaps more surprising given previous attempts to return—genuinely having fun.