Hi,
First of all, I'm not sure why your shell would be incompatible with 1.6.21... or is it simply that you are trying to install some .rpm/.deb/etc package?
The traditional way of installing any version of eggdrop, is to download the appropriate source, run the configure-script, resolve any missing libraries, and finally building the binary/module using make... If you can't find a suitable package for your specific distribution, this is pretty much your only option... (which is pretty much what the link posted by Caesar tells you)
Visit: http://eggwiki.org/Main_Page
and scroll down to:
Tcl related issues
In that section, find:
I installed the tcl-dev package but eggdrop doesn't find it on my new operating system (Ubuntu 14, etc.)
and click on it. Read that. See if you think that is at all about your problem with 1.6.21
Why do you want to install 1.6.21 though? Why not use the latest version?
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