why have you stopped supporting my mac for updates

Apple is not here and will never hear you complaint.

These ASC Forums are only fellow Apple user to Apple User forums.

As for being informed that a computer is not longer supported for updates. AFAIK - Apple has never formally informed individuals that a computer will not get any further updates.

It really comes down to Due Diligence on the part of the user to inform themselves just what is the most up-to-date this computer Qualifies to run in the way of macOS versions.

Generally speaking Apple has both upgrades, and updates. Updates you will find through Apple menu -> System Preferences -> Software Update.

Upgrades though are only available via the App Store. So each new fall (northern hemisphere) release of an upgrade brings out new hardware requirements. Apple on the specs page indicates the hardware requirements upon those releases. Generally after 6-7 years sometimes 8 a new release will no longer be available for the Mac in question. It doesn't mean you can't use a current web browser, but Apple's Safari is limited to the current and past two releases of Mac OS X upgrades.

http://www.macmaps.com/docs/DOC-13695/

is a FAQ* I made out of a user tip I have not been able to edit on Apple Support Communities since its last major revision, to cover what upgrades are compatible with which Mac. Date your machine, and then you can discover if you can run the newer OS or not. Keep in mind Catalina and later require 64 bit applications.

*Links to my pages may give me compensation.

What model/year is your Mac?

I'm sure there are information notices about Apple support. Maybe this applies:

Obtaining service for your Apple product after an expired warranty - Apple Support

You can be informed by looking at the Vintage and Obsolete section in Obtaining service for your Apple product after an expired warranty - Apple Support.