Was there any public discussion on some Slovak Linux/Unix forums about that missing key? Will there be any anger from people using that key for the pipe character?

Good question. I have searched slovak forums on this topic and there are people complaining on the missing bracket, but their resolution of the problem is to use different layout (Slovak). I decided to open this bug because I think missing closing bracket is a problem. Also I think that two pipe symbols bind to the same key are rather mistake than purpose. The group of Slovak users is too small and there are rather few people using extended backslash slovak layout. Not that this proves anything, but this shows how the key should look like (https://en.wikipedia.org/wiki/QWERTZ#/media/File:KB_Slovak.svg). So extended backslash layout should provide the keys: \ and | (direct keypress and with shift) and / and ) (direct keypress with altgr and with shift). This allows all current users to use | on the usual place and ) to use with altgr, which is not usual choice.

committed the fix to git, please check

I think "parentleft" is a typo...

A, I see you fixed it already.  Sorry for the noise.

Sorry for late reply. Took me some time to figure out where to find the patch and how to bring it to Fedora as I'm not linux developer, but I learned few tricks.

Correctly it should read parenright instead of parenleft (closing bracket, not opening). And also, as it appears, I was talking about <BKSL>. From my point of view, it should look like:
key <BKSL>  { [ backslash,        bar,        slash,     parenright ] };

thanks for  the amendment, committed.