Is D&D 6th Edition really coming in 2024?

In a word, "No".

In a few more words, "It's complicated... but perhaps not that complicated." The thing is, don't worry. Your investment in D&D 5th Edition won't become obsolete.

D&D "Little Brown Books" from 1974

First, let me take a moment to reflect. In 2024, Dungeons & Dragons will be 50 years old. This is a fairly mind-blowing anniversary. A vast majority of the player base these days is younger than the game itself. The legacy D&D has left on pop culture, video games, and tabletop gaming is practically immeasurable because it has literally influenced so many corners of the entertainment industry as a whole. 

The very first Dungeons & Dragons box set shipped in late January of 1974... and it's been a wild ride since then.

Now, getting back to "6th Edition"... This isn't a thing. 

During D&D Celebration, Wizards of the Coast announced they would be releasing revised core rule books in 2024. The revised books will update the character creation rules slightly to match what been published in Tasha's as well as include content from the books note above. Ancestries will replace Race... but they are still basically the same mechanically. Non-humans get some combination of +2/+1 in ability scores with their other special abilities and humans will use something akin to the Variant Human option. Ability bonuses will likely follow class or background rather than race selections.

Aside from character generation, all errata will be included and likely a number of spells will be altered or clarified. It is probable that some class paths will be tweaked, but I don't expect anything too stunningly different from class paths we've already seen in Xanathar's and Tasha's. We do know for certain that creature stat blocks are changing and we're seeing the first steps of that in Wild Beyond the Witchlight.

Historical Note: The AD&D 2nd Edition Revised covers removed
"2nd Edition" from the title graphic and downplayed "Advanced".

I've seen a lot of people refer to the upcoming updates as "5.5" which is a reference to when Wizards of the Coast revised 3rd Edition and printed the D&D 3.5 Core rule books. It's hard to gauge the accuracy of that take. The 3.5 rules did some fairly significant changes to combat, and moved the game much more toward an on-the-grid tactical combat (To be fair, third edition had started that trend, but 3.5 really cemented grid play as defacto standatd).

I'd hesitate to say we'd see sweeping changes. Calling it "5.25" might be more accurate, or perhaps just "5th Edition Revised" is more than sufficient. We've seem this before with AD&D. Second edition had new black cover revised books come out several years after 2e's introduction. It didn't really change the game much, but did offer some class changes in the form of "kits" and other skill option (which laid some of the foundation for 3rd Edition).

But the point it, Don't Panic. It's still D&D 5th Edition.