Using Classic Outlook on Windows for personal email, calendar, and contact management
I already save the customized ribbon as you suggest. I use it to restore my customized ribbon. The problem is that Microsoft keeps overriding the customized ribbon with the default ribbon so I have to restore it. This problem has arisen recently. For years it was not a problem. It does not seem to be a problem with Word, Excel, or PowerPoint.
Yes, the problem is much more pronounced in recent years and it is a real nuisance to constantly restore the ribbon from backups (as you noted this issue does not seem to affect other Office 365 apps). The focus on Outlook and pushing icons back on the ribbon seems to be part of a marketing strategy to push users to use the new "features"; it also may be part of broader strategy: there are persistent rumors that MS is ultimately planning to phase out the desktop Office 365 apps in order to push everyone to the web-based versions.
Phasing out the desktop version of Outlook will not fly with enterprise users. If that happens, we'll have to replace all the Office 365 desktop apps with something like LibreOffice.
I actually came here to find a solution to the ever-changing ribbons. I made ribbon changes in my mailbox, and had to do the same in a shared mailbox, only to see it resets my mailbox ribbon to the default. So annoying! I wish there was a way to stop Microsoft from changing my preferred setup.