Visual Deconstruction of Sentence Wanted

The core sentence is this:

those saw “art” therein.

A dummy subject it permits the writer to recast the "actual" main clause, those saw art, as the complement of the verb be in an it cleft:

It used to be that those saw “art” therein.

Therein is a preposition phrase complementing see, with there referring to the interminable forest of the language and explaining where they saw the art.

This is a temporal locative modifying that recast main clause, telling when this state obtained:

In the halcyon days of English learning in Japan,

This is a supplement glossing the temporal locative:

dating back as far as the Meiji Era,

This is a supplement glossing the main clause:

unlike recently,

And this is a further supplement, a relative clause glossing recently:

when, alas, much emphasis has been placed blindly on practical communicative “skills,”

This is a relative clause modifying (defining) those

who bravely chose to set foot in the interminable forest of the language

This is a supplemental ger-ppl clause glossing the original main clause:

finding the whole purpose and meaning of their noble endeavour in ... honing their “craft” .. or enjoying Shakespeare,

This his is a so .. that construction modifying honing their craft

so .. that they might dabble in poetical flourishes,

And this is a preposition phrase modifying the conjunct object of the preposition in, honing their “craft” .. or enjoying Shakespeare

in their second tongue.

The entire sentence is wittily contrived to exemplify the “dabbling in poetic phrases .. in their second tongue” which former Japanese students of English delighted in—a delight which the writer clearly shares!