Depending on the update channel, version (e.g. 2103, 2208 and such) are falling out of support. Each channel has different support lifetimes, e.g. Current Channel is till next version is released (e.g. when 2304 is publishes, version 2303 becomes unsupported), Semi-Annual is for 14 months since initial release.
Unsupported means two things: a) those version to NOT get security updates and longer, so devices on unsupported versions are becoming a security risk. And b) the connection to the Microsoft 365 services is not supported, so users may experience issues (performance, reliability, connectivity, ...)
Check this doc & video for more details:
https://learn.microsoft.com/en-us/deployoffice/updates/overview-update-channelshttps://youtu.be/eNn4PDkmo7sYou can check the update release overview for a quick overview of which versions are currently supported per channel:
https://learn.microsoft.com/en-us/officeupdates/update-history-microsoft365-apps-by-dateEspecially as unsupported versions are becoming an increasing security risk over time, we recommend to update those devices to a supported version. How do you manage updates today? Maybe switching to Servicing profiles is an option to automate update deployment? Doc & links to videos:
https://learn.microsoft.com/en-us/deployoffice/fieldnotes/adopt-servicing-profiles