Why pfsense 2.4 does not support nanobsd anymore?
It's working very good, why it's not been supported anymore?
Because it doesn't. Move on.
Short answer: elimination of technical debt.
You are free to fork pfSense and make a nanobsd version. Just don't call it pfSense.
It's working very good
It really, really wasn't. In very small sample sizes you may not have encountered many problems, but overall it had significant issues inherent to the platform. There was no way to properly fix it, and the hardware that originally needed NanoBSD has been EOL for a long time.
There are more detailed answers to this question on the blog, in various release notes, here on the forum, and elsewhere. It was a decision made years ago, there was plenty of forewarning.