Igst on import credit not taken in 3b

No you cannot claim. The IGST paid will be a lapsed credit.

why not. Igst on import paid to through cash

You should have taken the credit before 31-3-2019. For 2018-19, you have to take it before filing September, 2019 return or date of filing annual returns which ever is earlier. Else the credit will lapse

dealer paid tax through cash and offset liability when return filed, that time government get revenue, if tax paid to government Treasury and return not filed government can't claim that amount is a revenue.

In my view, there is no time limit for availing ITC in relation to import of goods.

Section 16(4) restricts ITC in relation to invoice or debit note.

ITC on import of goods is taken based on bill of entry and not based on invoice or debit note. Hence you can take the credit of tax paid on imports in 17-18 even after 31.3.19.