@Martin Horacek you are correct. Maybe the people at Boomi misunderstand that key difference between OAuth 1.0 and 2.0 (at least how Netsuite supports it currently). I would contact their support back letting them know this, and question it.

Reply from Boomi: Currently, the HTTP connector does not support HMAC-256 (OAuth). You can follow one of the below workarounds to resolve the issue. 1) Use NetSuite connector, as the article identifies that the NetSuite connector supports "HMAC-SHA256" currently. 2) Test with HTTP connector but use OAuth2.0. OAuth1.0 is very very old and currently does not support "HMAC-SHA256". This should be used/tested as NetSuite is transitioning from the older HMAC-SHA1 to HMAC-SHA256, as announced by NetSuite. 3) To implement "HMAC-SHA256" for OAuth1.0 is considered an enhancement and would need to be pursued with the assistance of your Customer Success manager. However note that this option would not be available in the next days or weeks, most likely longer.