Where to find OS update compatibility list for 2019 iMac?

Here is a very nice article showing OS compatibility for various Apple devices (for older hardware the OS list currently only goes back as far as 10.8 Mountain Lion):

https://eshop.macsales.com/guides/Mac_OS_X_Compatibility

The MacTracker app is nice since it provides a list of nearly all Apple hardware & OS versions along with many technical details for each model. Unfortunately MacTracker breaks down for current items since the app & database seem to be updated irregularly.....usually about twice a year if we are lucky so it may miss out on some of the latest hardware devices and newer point releases of the currently supported versions of macOS....definitely a good resource for older devices.

Its maximum OS version is Sequoia.

(262832)

➡️ macOS 13 Ventura is compatible with these computers (c.2017)

➡️ macOS 14 Sonoma is compatible with these computers (c.2018)

➡️ macOS 15 Sequoia is compatible with these computers (c.2018)

➡️ macOS 26 Tahoe is compatible with these computers (c.2020)

To upgrade to macOS 14.* (Sonoma) or 15.* (Sequoia), follow the directions in

How to download and install macOS – Apple Support

Or run the App Store application and search for "macOS Sonoma" or "macOS Sequoia" to locate the appropriate installer.

You’re welcome.