Check if this has already been suggested first
Nope, this is the first issue about it

What's your idea?
Add support for ico and icns into 1.12.2

Notes section under "Contributors and developers" of the readme

There are two versions of this mod:

• v2 which is the latest version that has been rewritten from the ground up. All work on new features is being done in v2.
• v1 is the original mod which has been backported to legacy Minecraft versions and gets maintained with minor bugfixes and changes.

So is this suggestion not considered a minor change? Looking at the code it seems the icon loader code is exactly the same, the only change this suggestion requires is a check for the ico and icns files. Or am I wrong about this, because i read both versions and couldn't find any major changes this needs

I added support for it myself and made a pr request here: #26

Thanks for your suggestion.

There are no plans to backport itlt v2 or its features to legacy Minecraft versions at this time.

Can I help you upgrade?

The readme of the project itself suggest that 1.12.2 is a current/maintained branch and not legacy.