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Photoshop ties the tool display to the workspace, and has been doing this for about a year.
If you use the Essentials workspace—or a custom workspace based on Essentials—all of the tools are visible by default. When you change the workspace—let's say you try out the 3D workspace—the tools that aren't part of a typical 3D workflow are hidden. You can always see the hidden tools by clicking on the 3Dot tool (it sits between the Zoom tool and the color chips), and you can always restore hidden tools with Edit > Toolbar, or by choosing Edit Toolbar at the top of the menu that is displayed when you click the 3Dot tool.
I wrote this for my students last year to help them understand this feature when it was released: https://www.rockymountaintraining.com/adobe-photoshop-cant-find-tool/
Essentials shows all the tools.
3D only shows tools that are part of a 3D workflow.
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To add to Barbara's correct answer, if you carry forward a custom workspace from a previous version, that included a specific toolbar set up, then the curvature pen tool will be hidden initially. Not surprising as it was not there to be included in the previous version, and just adding it could mess up your custom toolbar.
As you found, simply using edit and dragging it across corrects it - but don't forget to save the custom workspace - otherwise when you reset your workspace you will lose it again.