A potential "workaround" for some of us Outlook on the Web users for the time being, which by no means is ideal (in my opinion) and may not work well for your situation, but may work depending on your case:
You can add your shared mailbox to your primary mailbox (which should, as of posting, still allow you to access the MyTemplates add-in). See here for instructions: https://support.microsoft.com/en-us/office/open-and-use-a-shared-mailbox-in-outlook-on-the-web-98b5a90d-4e38-415d-a030-f09a4cd28207
*Some notes from my experience to be aware of before attempting:
- Each time you reply to an email, you must change the From address (to the shared email), and you'll have to add the signature, if you use a different signature for replies from your shared mailbox. Otherwise, the reply will be from your own email with your own signature, not that of the shared mailbox
- When you reply to an email from the shared mailbox via your primary mailbox, the reply is stored in your own sent items folder, not that of the shared mailbox—this may cause confusion for your coworkers as they won't see your reply or in the future can't refer back to your reply if needed
- Replies to emails sent from the shared address in your primary mailbox will show up in the shared mailbox as new emails. Replies to all emails in the shared mailbox will be shown as separate emails when viewed from your primary mailbox, not nested under the original email of which the reply was for
- If you make use of categories or labels in your shared mailbox, they won't coincide with the shared mailbox
- If MyTemplates breaks in our primary mailboxes again, this "workaround" will be void lol
- Should you want to attempt this, try a few test emails to see how the experience will be before sharing this as advice with your coworkers who might get confused by the additional steps required
If those considerations still make this "workaround" worth it to you, then once you have added the shared mailbox into your primary mailbox, on the left sidebar open whichever folder under the shared mailbox emails you need to reply to will be stored (usually the inbox).
- Select the email you wish to reply to from the appropriate shared mailbox folder. Or compose a new email.
- Change the From address to the shared email. The first time, you may have to manually add it ... click the From field and Change address, then type and select the shared email address. If the From field does not show, go to your options and select Show from.
- Update the signature. If needed, add your shared email's signature to your saved signatures so that you can easily add it to email replies in the future.
- Open the MyTempltes add-in and, as you would have in the past, select the template you wish to fill into the email. You may first have to add all the templates into the add-in. You may first need to go into the Get Add-ins icon and select/add the MyTemplates add-in to your primary mailbox.
- Compose the email as desired.
- Double check that the From address is the shared email, and that the signature appears as desired.
- Send the email.
Hopefully, this may serve useful for some of you who have simple situations with your shared mailboxes and don't need your replies to be visible to or stored in the shared mailbox itself. BUT BETTER YET, LETS HOPE MICROSOFT FIXES MYTEMPLATES IN SHARED MAILBOXES FOR CHRISTMAS!!!!!