be it lengthy retreats dedicated to a specific task.

这是一种省略式倒装句。省略了连词whether 的“让步状语从句”：

whether it may be/is lengthy retreats dedicated to a specific task.

it 指前文的 the art of deep work。

be it... 是古英语遗留下来的一种倒装式虚拟语气，be动词用原形。又如：

She enjoys watching dance, be it ballet, national minority dancing or any other sort of dancing.

(= whether it is ballet...)

芭蕾舞也好，民族舞也好，随便什么舞她都喜欢看。

来自《现代汉英综合大词典》

Speak frankly, I pray you, be it for life or death. " 

(= whether it may be/is for life or death)

我请你坦率地讲出来, 不管我是该活还是该死。”

来自英汉文学 - 红字

Home is home, be it ever so homely.

(= whether it may be/is ever so homely)

家就是家，不管它是多么平凡。