Just reached lvl 50 Ninja. Is this the endgame?

[REPROBEAN_CHILD]

You'll want to at least finish the 2.0 story (ending with Praetorium) as a LOT opens up. After that it's mostly unlocking dungeons, working on your relic (no reason not to get the ilvl 90 version, though it's understandable to stop there), and gearing up. You'll also want to continue through the story for various other unlocks (you generally need to be on top of the story and major sidequests to do extreme and trial roulette, two of the better tomestone methods.

What about the alternate classes? I'm working on them, but does anything change or them?

[Urthdigger]

What about the alternate classes? I'm working on them, but does anything change or them?

No. "Endgame" refers to certain content that you can only do after beating the main game. This includes most post-50 gear progression, and higher difficulty raids and trials. Leveling other classes is something you can do at any time, even before hitting level 50, so it's not endgame. Endgame just unlocks new content, it does not modify existing content. The only difference between it and any other quest to unlock content is the quantity of content unlocked.

[SecretCrowds]

What about the alternate classes? I'm working on them, but does anything change or them?

I would suggest you hold off leveling other classes and focus on completing all the main story quests first.