http header names that contains _(underscore) are not displayed.

[DeslogesB]

Request containing http headers with _ (underscore) in their names are not displayed in the Headers debug page.

Even if _ is not common, it seams to be accepted / known by major actors :
https://developers.cloudflare.com/rules/transform/request-header-modification/reference/header-format/
https://techdocs.akamai.com/property-mgr/docs/strict-header-parsing
https://www.ibm.com/docs/en/sva/10.0.2?topic=stanza-http-header

Following nginx Doc, they are valid but not enabled by default : https://www.nginx.com/resources/wiki/start/topics/tutorials/config_pitfalls/?highlight=underscore#missing-disappearing-http-headers

What about managing them in webhooksite ?

[fredsted]

This is supported on Cloud as of 13 March 2024: https://docs.webhook.site/news.html#13-march-2024

For the OSS version, PRs are welcome.