Hi, is it legal to use a client ( like BadLion Client) for some use fulll features. For exemple the toggle run or being able to use mouse button 4 and 5 in the inventory ( only for old versions as 1.8.9)

Badlion is banned.

Ow... Okk, hopefully i havent made a good time until now. Thx

You can use optifine or jellysquid for performance improvements.

only use optifine for versions pre 1.16

Why cant I use Badlion ? What's the problem with it, just to know

Well it's free, and pirated. We don't have rules against it but it's still illegal to use Tlauncher, BadLion, etc. as it is illegal to pirate anything. Especially if it costs money.

"Why cant I use Badlion ? What's the problem with it, just to know"

It is a modded client. Modded clients like Badlion, Lunar Client etc. are banned.