This is a common prompt seen among many Slack users, which occurs because the Slack app does not have ample permissions to execute an update's installation.
Blocking Automatic Updates (Requires MDM)
The most efficient way to suppress this prompt is by blocking automatic Slack updates with a Custom Profile. This will prevent end-users from updating Slack themselves, but will block the prompt entirely.
To keep Slack up to date while enforcing this profile, updates must be deployed via Addigy - you can use PreBuilt Apps to accomplish this. See Prebuilt Apps in Policy for more information.
We recommend configuring this Custom Profile using iMazing Profile Editor. Search for Slack in iMazing, then select the 'Slack Client Management' payload.
De-select the Auto Update setting within the payload. Ensure the box is un-checked and there is a blue dot next to the setting (which indicates it has been configured), as shown in the screenshot below.
After saving the Custom Profile as a .mobileconfig file, refer to the following documentation to deploy the profile to your devices via Addigy: How To: Configure and Deploy a Custom Profile