This is not an encoding problem, but the IRC colour codes.

I am wondering why don't you just html interface http://tools.wmflabs.org/wm-bot which display these as colors I don't see any point in removing these from txt files as some programs can render them (for example mirc can load these txt files and show colors as they were)

Just because the topic on IRC points the links above ("Channel logs: https://bit.ly/11g3AEP") and there is no way to know that there is an HTML version.

That is somewhat true, but unfortunatelly I can't put any header to txt files. You probably need to update the topic. Information about html version is on [[meta:WM-Bot]] for sure.

I wonder who is going to fix the topic in each of the channels affected by this...

I'm still not clear exactly what the problem is in this ticket. @He7d3r what do you hope to gain or have changed by this ticket? If you don't want the color codes, why not load the .txt in a text editor and do a simple text replace to strip them out?

I want
http://bots.wmflabs.org/~wm-bot/logs/%23wikimedia-dev/20150112.txt
to be as readable as
http://bots.wmflabs.org/~wm-bot/logs/%23mediawiki/20150112.txt
for example.

Downloading a file, opening it in a text editor and doing a search replace just for reading a few messages doesn't seems to be a good approach, because it would be a lot simple to just follow the link to the online copy.

Ok, in that case you need to open it using http://tools.wmflabs.org/wm-bot which would display these in a readable format. It's easy to strip color codes from txt files, but not possible to recover them. So I prefer to have them there, so that we can get both original and stripped messages.

It's generally a job of local channel operators to maintain topic, if they aren't able to do that, they should probably get more operators, there are hundreds of users across wikimedia channels that would happily volunteer.