Do applicants for the DTV through soft power need to practice Muay Thai for five years?

You'll need to update your documents with immigration when extending your stay in Thailand. So far, when crossing the border, it seems the vast majority of people don’t need to show anything. However, balance your risk as immigration has full authority to perform checks at any time (and remember what are their conditions for extension). Don’t rely on those who claim that checks will be "announced" that’s pure nonsense. Checks have already been conducted on other types of visas (student, volunteer, etc.). My best advice is to check visa forums where long-term expats share real experiences with immigration.

If you sign up for 5 years of Muay Thai, they will probably roll out the red carpet for you! 😅

Up to now you don't have to show anything on re entries. The people who will have problems are those who try to live in Thailand for the full 5 years without spending significant time away. But there are ways around that.

for the application not. for extension you need ongoing activity. i bet down the line you will have to proof some ongoing activity sometimes. they could start checks on entry. same as they did in the past with ED or volunteer visa… to believe that there will be no check for 5 years is pretty crazy and doubtful