Good afternoon!
Welcome to the Qiime 2 forums. Let me see if I can help answer your question.
While I couldn't find the old 1.4.3 version of matplotlib on conda, I did find it on GitHub. Maybe you could install it from there:
Keep in mind that this version of matplotlib is over 4 years old, so it might be time for an upgrade! Have you considered using Qiime 2? :qiime2:
Colin