A 1.21.10 server runs on Minecraft 1.21.10. That usually means you connect with a 1.21.10 client and the world behaves exactly like 1.21.10: the same blocks and items, the same mob behavior, the same redstone timing, and the same bugfix outcomes.
In practice, the version is the contract. It is the difference between a farm that stays stable for months and one that breaks because the server is translating packets or mixing mechanics across releases. Players who build technical, run survival economies, or just want vanilla to match singleplayer tend to prefer a strict 1.21.10 environment.
The tradeoff is lock-in. Anything layered on top, plugins, add-ons, datapacks, still has to fit 1.21.10, and some servers will update only when their stack is ready. When a server says 1.21.10, the point is consistency: what you learn in 1.21.10 is what you get online.
