Learn English – Had been vs was

The meaning is ambiguous, but here is one possibility:

Had this been a solution that could be provided in Java 5 and above, the overloaded "replace(charSequence, charSequence)" could have been used.

The tenses are inconsistent in the first example, and "Have this been..." in the second example is not normally used.

The construction is unimportant ... any more is not generally accepted. As noted in wiktionary (and amplified in answers to the question What are the possible meanings of positive any more Peter Shor mentioned), any more is used:

1. In negative or interrogative constructions: from a given time onwards; longer, again. (eg) They don't make repairable radios any more.
2. (colloquial, chiefly Northern Ireland, US) In positive constructions: now, from now on. (eg) ‘Quite absurd,’ he said. ‘Suffering bores me, any more.’ (DH Lawrence, Women in Love)

To retain the slight sense of change in your original example, one can replace is with has become near the beginning of the sentence:

This has become unimportant for the younger generation.

You can use now as previously suggested ("This is unimportant for the younger generation now"), but note that unless a contrast has been made, now appears superfluous; the present-tense statement "This is unimportant for the younger generation" will have the nearly same meaning.