Microsoft Publisher - others have ask about this but there is no visible answer from Microsoft

No supported side-by-side configuration is documented for using Microsoft 365 Apps and Office 2021 on the same Windows device when they include overlapping apps such as Publisher, Word, Excel, or Outlook.

The supported guidance in the available Microsoft Q&A content is:

1. Microsoft 365 Publisher ends for subscribers after October 1, 2026. After that date, Microsoft 365 subscribers will no longer be able to open or edit Publisher files in Publisher.
2. A perpetual version of Publisher, such as the one included with Office Professional 2021, can continue to run locally after 2026, but support for that perpetual version ends in October 2026.
3. Running Microsoft 365 Apps and Office 2021 together on the same device is not considered a supported setup when the installations overlap. Reported issues include activation conflicts, shared installation behavior, or one version overriding the other.
4. The most reliable approach is to keep them separate, such as:
   • Microsoft 365 on the primary device for current Microsoft 365 apps and services
   • Office 2021 with Publisher in a separate environment, such as a second PC or virtual machine

If checking an existing Office 2021 installation, the documented way to confirm what is activated is:

1. Open Publisher or another Office app.
2. Go to File > Account > Product Information.
3. Verify whether it shows Office Professional 2021 or Office Professional Plus 2021 under the activated product.

That confirms which product is activated, but it does not change the side-by-side support limitation.

References:

• Microsoft Publisher will no longer be supported after October 2026
• Office Professional 2021 use in tandem with 365 - October 2026 - Microsoft Q&A
• Standalone Publisher / Office Professional Plus 2021 - Microsoft Q&A