Can restaurants serving liquor claim 100% itc ?

No...

Regular scheme registration
input tax credit is possible only when there is a valid tax bill and there are other aspects.

Supply of liquor is Non taxable supply , & In term Section 17(2) R/w rule 42 ,the credit of ITC restricted to ITC attributable to Taxable Supply. Hence need to reverse ITC in proportion to supply of liquor.

Eg : ITC on purchase of Glass need to be reversed in which liquor has been served.

(Kindly refer Rule 42 & 43 )

Liquor and fuel both are not subject to gst. It's a taxable supply.

yes, not under the purview of gst hence not taxable in GST.