A long term stagnating economy and especially a shrinking one would be considered an economic disaster anyway. The key is to avoid social problems due to shrinking the economy. There are plenty of ways of doing that, but propably the most important one is to limit working hours. That means earlier retirment, less hours worked per week and more vacation. All of that are things mandated by countries before. With a globally stagnant population that might very well happen. In fact you see falling working hours in Europe over decades.

Degrowth basicly tries to limit change the size of the economy to one, which stays within planetary boundaries. One of the most important ones of that is certainly greenhouse gas emissions. So something like the emissions trading systems of the EU are inherintly degrowth policies. They cap emissions and lower that cap over the years. Some industries are already in some trouble due to that, namely the coal electricity industry, but a lot of other heavy industry in the EU has problems as well due to that policy right now.

Another big one is increasing wild space, which the EU also has passed a law, forcing its members to rewild a few percent of their land. That is limiting resource use for sure.

There are also a lot of other enviromental regulations doing similar things. The problem is not moving fast enough and still believing in economic growth, but there are policies, which go very much in the right direction and are more then just token policies. Obviously a lot more is needed, but change within the current system is certainly possible. However you are right, the fear of a drop in gdp has to go and be replaced with a the idea of improving quality of life. Obviously having a strong economy helps with that, but still it should be an option, if the econmy shrinks.