Learn Russian: Заказчик волнуется насчёт дедлайна. - The client is worrying about the deadline.

While волнуется о дедлайне is also perfectly correct, насчёт translates more closely to regarding or concerning. It is very common in everyday and business Russian to pinpoint a specific issue. Remember that unlike о which takes the Prepositional case here, насчёт always requires the Genitive case, which is why we see дедлайна.

Is дедлайн just the English word deadline, and does it decline like a normal Russian noun?

Yes! Modern Russian corporate and business language heavily borrows from English. Because дедлайн ends in a consonant, it is adopted as a standard masculine noun. Therefore, it takes the standard masculine Genitive ending -а after насчёт, becoming дедлайна.

What is the exact difference between заказчик and клиент?

A клиент is a general customer, like someone buying goods or using a standard service. A заказчик (from the verb заказать, meaning to order) specifically refers to someone who commissions a custom project, service, or piece of work. Since there is a deadline involved for a deliverable, заказчик is the most accurate word here.

Why does the verb волнуется have the reflexive -ся ending in this sentence?