I'm using st terminal and I can't get rid of bold text, my curent font support bold and apply this patch doesn't help: https://st.suckless.org/patches/xresources/
with st.bold_font: 0 in .Xresources

Howto disable completely bold font with st?

Patch

Unfortunately no one has made a patch for the current version (v0.8.1).

Fork

If you don't want to change the source code, then you can try the fork xst which has the ability to disable bold fonts, but also includes other patches.

Fontconfig

Another option (more of a workaround) is to disable bold monospaced (or specific) fonts everywhere by creating a config file for Fontconfig, for example in $HOME/.config/fontconfig/fonts.conf, or add to your existing, the following:

<?xml version='1.0'?>
<!DOCTYPE fontconfig SYSTEM 'fonts.dtd'>
<fontconfig>
 <match target="pattern">
 <test qual="any" name="family">
 <string>monospace</string>
 </test>
 <test name="weight" compare="more">
 <const>medium</const>
 </test>
 <edit name="weight" mode="assign" binding="same">
 <const>medium</const>
 </edit>
 </match>
</fontconfig>

If you add to an existing file, make sure to remove the first three lines and the last line.

This assumes that you have compiled st with something like:

static char *font = "monospace:size=13:antialias=true:autohint=true";

In the config.h. Also that you have defined the font you use somewhere in your fontconfig something like this:

<alias>
 <family>monospace</family>
 <prefer>
 <family>Px437 IBM VGA8</family>
 </prefer>
</alias>
<alias>
 <family>Px437 IBM VGA8</family>
 <default>
 <family>monospace</family>
 </default>
</alias>

Then run the program fc-cache. Note that this will disable the Xresource colors color8 through color15 in st, but the patches might have done that as well.

Fontconfig can be a very difficult thing to configure, and it is very easy to make it do something that was not intended. Therefore, to cover this fully, is beyond the scope of this answer.