It seems to be a beta/recent feature on Github, but today I learned we could highlight a blockquote by adding its "types" in the first line:
> [!NOTE]
> Useful information that users should know, even when skimming content.
> [!TIP]
> Helpful advice for doing things better or more easily.
> [!IMPORTANT]
> Key information users need to know to achieve their goal.
> [!WARNING]
> Urgent info that needs immediate user attention to avoid problems.
> [!CAUTION]
> Advises about risks or negative outcomes of certain actions.This input will produce the following output:
This might work in every single place as you can write markdown such as READMEs, .md files (that will be rendered in the GitHub platform), comments, etc.
Update
This feature is there for a while already and it's called "Alerts". Check the documentation link in the resources section.