On which wiki have you done that change?

@Trizek-WMF It's not a WMF wiki, but you can observe the change here: https://test1.miraheze.org/wiki/MediaWiki:Flow-terms-of-use-edit

Also note that standard wiki-markup such as ''' for bold also doesn't work, only wiki links ([[Page]]) work.

Ok, thanks!

@Etonkovidova, can you have a look at it? I can't test on test wiki.

@Trizek-WMF I asked @AlvaroMolina since they are an administrator there, and they have edited the page on test.wikipedia.org, see https://test.wikipedia.org/wiki/MediaWiki:Flow-terms-of-use-edit.

Thanks! The result is <h3>By saving your changes, you agree to the [[terms of use]] for this wiki.</h3>

@Trizek-WMF: I have given you the administrator permission in Test Wikipedia so that you can work more comfortably with the resolution of this task.

In T174165#3553105, @AlvaroMolina wrote:

@Trizek-WMF: I have given you the administrator permission in Test Wikipedia so that you can work more comfortably with the resolution of this task.

Sweet! Thanks @AlvaroMolina.

I4ve changes <h3> to bolded wikitext, but it doesn't work.

I checked the issue in betalabs - yes, the markup will be displayed. When it's debatable whether or not the formatted text should be present in such elements, markup tags should be either filtered or detected upon saving the edits and not saved.

It is present for generic MediaWiki system messages and other extensions, so I don't see why it shouldn't also be the case for Flow.