We currently support both plural and singular forms with sections headers so that both

*** Test Cases ***
Example
    Log    Message

and

*** Test Case ***
Example
    Log    Message

are valid.

The plural form is used a lot more widely. The main reason the singular form has ever been supported than in the early days Robot's data was in HTML tables and tables typically had headers like in this example:

Nowadays there are no concrete benefits in supporting both header formats. There are, however, some drawbacks:

• Supporting multiple formats like this makes the overall syntax a bit more complicated to learn and for external tools to support.
• It makes translation effort (Add and document translations #4390) a bit bigger.
• As pointed out in this comment to Add and document translations #4390, some languages support also a dual form that we should support if we support both singular and plural forms.

Adding support for the dual form wouldn't be a big task, but it would make the overall syntax even more complicated. A much simpler solution is supporting only the plural form. We can consider the section header like Test Cases to indicate that "below this header we have tests" and it works fine regardless how many tests there actually are. We have thus made a decision to:

• support only the plural form with translations (covered by Add and document translations #4390), and
• remove the support for singular form with English.

We obviously cannot just remove the support of singular headers in RF 5.1 because the change would be backwards incompatible. These are the steps we are planning to take instead:

1. Document that singular headers are deprecated in RF 5.1 release notes and in the User Guide. No visible deprecation warning yet. This is covered by this issue.
2. Start emitting deprecation warnings. This can earliest be done in RF 6.0, but we don't have too big hurry and waiting for 7.0 might be better. This is covered by Loudly deprecate singular section headers #4432.
3. Actually remove the support for singular headers. This can earliest be the next major release after the one where they are visibly deprecated.