Dear Daniel,
Good day!
Thank you for posting to Microsoft Community. We are glad to assist!
Based on your description regarding "I'm currently using the new Outlook, how do I easily insert a horizontal line in an email, now that that feature has been removed the "Insert" menu?". After thorough research and testing I found that you are definitely right. This particular setting only exists in the classic Outlook and is not yet available in New Outlook for Windows at this moment. The New Outlook is in a Preview state, and they are still working on the platform/foundation of the application itself. At this stage of the development, luckily If you heavily rely on this feature, you may consider toggling out of the new Outlook for Windows preview for now.
However, thank you so much for bringing this to our attention, your suggestion is very helpful, and I believe it could make the use of New Outlook even better. We will surely raise your concerns to our Product Development Team and hopefully this can be included in the future updates. In the meantime, we sincerely recommend you post your concern as an idea to our Outlook feedback site via Outlook · Community (microsoft.com) our Product Development Team are committed to enhancing the customer experience by continually examining suggestions to include in the design and development of future product upgrades based on your suggestions. Hopefully your suggestion will help many other people in the future. Customers’ feedbacks are always valuable for us. Thanks for your precious time and your understanding would be highly appreciated. Have a nice day!
Best regards,
Inema |Community Moderator