Outlook keep resetting ribbon back to default after changes

That solution won't work for those with multiple mailboxes. Reason being is that you can only change one mailbox, and it will revert if you try to change another mailbox (under the same Outlook profile, of course). I suppose I could change one mailbox, lock down the files, but then I can't change another mailbox. I wish Microsoft would just fix this customization thing to work right, and not force anything upon us.

I'm not sure what you are meaning by 'multiple mailboxes under the same Outlook profile'. I have different profiles for my different email accounts. If I change the ribbon in one profile, when I switch to another profile the changes stick. How does your setup work?

That solution won't work for those with multiple mailboxes. Reason being is that you can only change one mailbox, and it will revert if you try to change another mailbox (under the same Outlook profile, of course). I suppose I could change one mailbox, lock down the files, but then I can't change another mailbox. I wish Microsoft would just fix this customization thing to work right, and not force anything upon us.

After I added this entry to the registry, my Ribbon changes stuck.

That works if you only have 1 mailbox. You see, before you set up the DWORD, any changes to 1 mailbox stays. After the DWORD, the same mailbox keeps your changes. But, when you go to a 2nd mailbox, you try to make the changes there, and it immediately reverts it all back, as if that DWORD means no changes allowed. Without the DWORD, you can change 1 mailbox, go to another mailbox, make the same change, and the 1st mailbox gets reverted. DWORD prevents that reversion, but also prevents you from making more changes. So, not exactly useful for people like me (I have multiple mailbox, e.g., 1 for myself, 1 for support at Company 1, another for support at Company 2, and some shared mailboxes.)

Well, it turns out that I was overly optimistic. Today I had a couple of icons graciously shoved back on the inbox ribbon by the all-knowing managers of Microsoft (they obviously were quite sure that I had removed the icons by mistake and desperately needed them...). Fortunately this only happened on the inbox ribbon, none of the others were changed (for now).

Nevertheless the frequency of MS invasiveness is MUCH lower than before so I'll keep the registry entry for now and keep the form updated

I have this same issue, except that my customized settings will not even stay for 10 minutes! As soon as I click on "reply" or 'new email" then back to home I have lost my customizations. Then I go from home back to "reply" and my settings there are gone and I have all this OneNote, Viva Insights, Templates, Polls and Immersive GARBAGE shoved back in my face. I do realize that all of these things help MS collect user info (for their own marketing reasons) but it's really annoying that as much as this software costs my company (with 100s of employees using MS 365) it isn't even easy or productive for us to use it. How about actually making products FOR THE USER instead of your own data gathering.

You might try manually disabling Office Updates (File/Office Account/Microsoft 365 and Office Updates/Update Options). For me this has seemed to reduce the frequency ribbon of icons being pushed back to the ribbon, but hasn't gotten solved the problem completely. While this is better than nothing like most workarounds it is a nuisance.

If you do try this don't forget to re-enable Office updates from time-to-time, occasionally there are some that you actually need, especially security updates. I re-enable Office updates once a month just before the first Tuesday of the month when non-security updates are released; after updating I immediately disable them. That way I have a whole month of quiet while Microsoft is busy releasing the inevitable patches for buggy updates. I use the same procedure for Windows updates as well.

Apparently Microsoft releases non-security updates on the first Tuesday of the month and security updates on the second Tuesday. For Windows I update everything just before the first Tuesday of the month.