Starting last night it is no longer a request/recommendation, but a legal requirement for both testing and isolation after entry into Denmark from abroad.
Failure to do so can be fined by 3500 dkk.Source: en.coronasmitte.dk–
At entry by air you need a fresh test before boarding and another test will be performed at landing before exit from the International Airport.
At entry by sea and land a fresh test before entry to Denmark is required. Don’t expect testing facilities at all times by the border.
The new things upon entry are:10 days MANDATORY isolation 2 negative tests AND 10 days before breaking isolation. These rules are for everyone, Danish residents included, though Danish citizens cannot be refused at entry, but they can and will be refused access to flights for Denmark.