I'm reposting my feature-request from the Follow feature announcement as I was asked to by @YaakovEllis in a comment:
Can you please add this as a feature request in a separate post. No longer tracking things here.
Please add the ability to follow deleted posts; from this comment it appears it was planned (the button appeared) but dropped:
followbutton is no longer available on deleted posts – Yaakov Ellis♦
There's a couple reasons why one may want to follow deleted posts which they weren't following before (so not related to this current bug): checking if the author revamped the post, to possibly cast an undelete vote on it, etc.
Related support question: if I follow a post that is later deleted, do I get a notification about it? I'm not sure if deletions are covered by the "notices":
you will get inbox notifications for all new answers (in the case where you followed a question), comments, edits, and notices.