Like JSU on Jira Server, Jira Cloud proposes a "Field required" validator in transitions.

However, when fields are set as required in the workflow, they do not appear differently from optional fields in the screen.

It is a big step backwards, and it can lead to confusing results.

1. JSU on server proposes the best option: a red asterisk appears next to the field name.
2. Jira Cloud could propose the same functionality

Some community members suggest setting up directly a red asterisk or a "required" information in the screen name, but this workaround is way too rigid is some business cases.

Screenshot showing a field ("Description (cf)") that is required by a workflow validator. Before clicking Create:

After clicking Create:

At no point is the required field visually highlighted.

Workaround

Making the field required by specifying field behaviour in a field configuration (instead of via workflow validator) produces visual indicators. 

Before clicking Create, there is a red asterisk:

 
And if you click Create without filling in the required field it will be highlighted like so: