iMac Target Display Modus does not work

You can't use Target Display Mode with a Windows computer, and you can't use Target Display Mode with an iMac later than a 2014 model, or with any macOS later than macOS 10.13.6: High Sierra

See: Use your iMac as a display with target display mode

And as far as I know you can't use an iMac as a monitor. You'll have to buy a monitor for your PC laptop.

I get the impression Apple doesn't want to support the feature any longer

More than an impression - Target Display Mode is history. "Target display mode is a feature available with certain older Mac models."

the article you are referring to is about Mac to Mac.

Which is rather the point. To reiterate my comment - you can't use Target Display Mode with a Windows computer.

they recommend to use newer keyboards...

Not really: It might also help to use the keyboard that came with your iMac (ie. an iMac that can perform Target Display Mode) - which is specified in the support document as iMac models introduced in 2011, 2012, 2013, and mid 2014; and 24-inch and 27-inch iMac models introduced in 2009 and 2010.

stevenvanbauwel wrote:

I would recommend to Apple development and product release teams to inform their resellers and external support sites well ahead of new coming upgrades, particularly about functions that Apple want to take out of support.

TDM was deprecated years ago. It wasn't a secret. The last major release to include TDM support was High Sierra, released in 2017.

Apple doesn't have any "external support sites". Apple is not responsible for making sure that every single website on the Internet gets their facts right. That's just ridiculous.

It almost certainly will not work, however please look over Target Display Mode (Target Display Mode) to see if you own Macs that will work and you are using the correct cables and have done basic troubleshooting such as restarting Safe Mode (both Macs), checking versions of Mac OS installed on each Mac etc..

Support for TDM does not exist in any version of macOS newer than High Sierra.

Even if you could get an iMac that supports TDM to cooperate with a Windows PC, You can't do it if the Mac is running Catalina.

I would recommend to Apple development and product release teams...

They won't see your concern here. There is no way they can review 100s of thousands post per day worldwide. If you really want an Apple employee to see your concern, use this feedback link:

Feedback - iMac - Apple

Please consider these facts:

1) Users, not Apple, accidentally discovered that a limited number of 2009 and 2010 27-inch iMacs would work as an external monitor with some non-Apple products. It was never a "sure thing."

2) Apple never intended, documented nor supported this "happenstance."

3) That quirk of video hardware stopped when the 2011 iMac models appeared. NOTHING newer that 2012 even accidentally supported non-Apple devices through Target Display Mode.

4) The last iMacs that could be used as an external monitor for another Mac were discontinued in Late 2014

5) People are spending a lot of money on unusable cables and adapters based on misinformation posted on other web sites. If you wasted money because of bad advice from a non-Apple web site, you have a legitimate complaint with that web site.

There is probably more spurious information on the internet about Target Display Mode than there is about the Loch Ness monster and a flat Earth. TDM has been incorrectly attributed as doing everything from supporting any video device ever made to parting the Red Sea.

Apple did nothing wrong. In 2014 there was almost no demand for TDM. Web "journalists" failed to be diligent in researching the subject before puking forth inaccurate "articles."