As we have been flagging for some time, Microsoft will no longer be supporting their Windows 10 operating system from October. They've now announced that the other 365 apps (Word, Outlook etc) will be subject to the same fate. Support for these will be withdrawn on the same day.
What does this mean for still those using both WIndows 10 and 365 on that day? Nothing on day one - both will continue to work. However, that doesn't mean you should ignore this deadline. As with the end of support with Windows 10 itself, Microsoft 365 apps are frequently targetted by people trying to compromise your system due to their ubiquity. If you continue to use an out of support version that is no longer receiving security patches, there is far more chance a maliciously crafted Word document or similar will be able to deliver its payload and wreak whatever havok it is designed to enact.
There is only one effective remedy to this and that's to upgrade to Windows 11. Depending on the age of the machine, that may not be possible, in which case you'll need to replace that machine. Certainly from a business perspective, given something like ransomware has the ability to bring work across a whole company to a stop for days at best, and weeks at worst, running out of support software just isn't worth the risk. Running supported software doesn't remove these risks entirely, but it is an essential line of defence.
The eventual problem in not switching will be that files generated by those that are using a version that is receiving updates will ultimately not work properly to some degree or another on the out of date version, and vice versa. Again, this will not be a day one problem, but a slow burn that will compound over time,