HDMI 2.1 - Do cables longer than 5m have to be "Active" cables to be true 2.1 standard? Are "Passive" HDMI 2.1 cables over 5m useless?

[Euphoria]

I'm really getting confused, I've barely seen any details about HDMI 2.1 so I'm completely out of my depth.

 

Googling tells you HDMI 2.1 can go up to 20 or 25m but then forums and some tech websites say above 5m it has to be an Active cable otherwise it's fake or at least not true 2.1 standard.
If it's a passive cable above 5m then it's actually going to be a compressed signal, it might physically output an 8k/4k120Hz signal but it won't have full HDR1000 or 10-bit color... or something like that.
But then others say differently, that passive is fine up to 10 or 12m.

 

What is the truth behind this?

[Euphoria]

I should probably add the reason I need to be sure is I need a 6 to 8m HDMI 2.1 cable for a LG C2 display, so it needs to be 4k 120Hz.

Looking at cables on Amazon the 1 to 5m cables seem to be marketed as HDMI 2.1, but then as soon as you go above 5m they seem to drop the 2.1 prefix and instead say ultra high-speed and things like that.

 

Then I found 10-15m 2.1 cables that are marketed as true 2.1 but they say they are active and cost $100-200. 

[Glenwing]

A cable that has passed the Ultra High Speed certification and has a genuine holographic label on the package is a "true" UHS cable. Cables that do not have the certification are not true UHS cables. It's not based on length or passive/active.

 

https://hdmiforum.org/specifications/ultra-high-speed-hdmi-cable/

[Euphoria]

Oh that's fantastically helpful, thank you.