When is it okay to not use desu?


I've been looking around for this, but all I can find is pretty basic stuff
about desu. I've studied Japanese, but writing texts in Japanese still stumps
me sometimes. Right now I'm wondering about the use of desu, as in this case:

> 「仕事場での高い生産性と楽しい1日は、使い心地が 良い道具を揃えることから。」というのは姉妹の口ぐせ。
> 機能的でかつインスピレーションあふれるワークプレイスを一緒に作ってみませんか。アンナのフェミニンでシンプルなデザインが、上品な印象を与えるコレクションの登場です.

The text is from a product page, which can be found here:
<https://sostrenegrene.com/ja/campaigns/%E3%82%AA%E3%83%95%E3%82%A3%E3%82%B9%E3%81%A8%E3%82%B9%E3%82%AF%E3%83%BC%E3%83%AB/>

How do I know I can leave desu off the end of a sentence and still make it
sound proper. Why is 'というのは姉妹の口ぐせ' without desu, but 'コレクションの登場です' is with
desu? Is there a set of rules for this?

As of now, I've written texts filled with 'masu' and 'desu' at the end of all
sentences, and it sounds a bit rigid. How do I write a more natural sounding
text?

