old iMac needs Word and Excel software

ms says office 2016 support all macs with intel cpus https://support.microsoft.com/da-dk/help/4026853/office-office-2016-system-requirements

office 2019 og office 365 are only supported in the latests 3 macOS versions https://support.office.com/en-us/article/microsoft-os-requirements-for-office-2019-for-mac-and-office-365-for-mac-16b8414f-08ec-4b24-8c91-10a918f649f8?ui=en-US&rs=en-US&ad=US

if you mean the apps from app store then they are without an office 365 subscriptions only viewer and can't edit or save office files

What is you iMac sub-model? If new enough it may be possible to upgrade your macOS version to make newer versions of MS Office possible for you. Find your sub-model by doing "About this Mac..." to get this window:

Keep forgetting this option.

https://products.office.com/en-us/free-office-online-for-the-web

Free to use, but all of your data is saved on Microsoft's servers. But you're using the latest Office 365 through your web browser, negating the need for the latest OS to use it.

What I don't know is how well it will run in an older version of Safari. You could try Firefox if that doesn't work. I also don't know the full details of using the free version, but I would assume you would be required to create an MS account.

By purchasing a copy. Office 365 and 2019 will not run in El Capitan. Sierra is the minimum.

The newest release you can run in El Capitan in Office 2016.

Check the free online version I mentioned to see if that works. Well, and you don't mind all of your personal data being on MS's servers.