That's not legal. To work in the EU you need a work visa. Though they ignore the business trips. What's legal and what's checked are different things.
E.g. new foreign colleagues need to get a tax id in The Netherlands before they do any work. Doesn't matter if they came from the US.
Of course, there are a lot of unenforced laws on the books, but I'm a little skeptical that the millions and millions of people who travel cross-border on business trips every year are all mostly breaking immigration laws.
To be fair, my experiences are all on relatively short trips--a few weeks at most and usually less. Perhaps there would be more issues if I were staying 90 days and was open about workationing the whole time.
Attending conferences is usually ok, but it's a complicated topic because it depends on the traveller's passport, the visited country and how strict rules are enforced.
As a foreign national, for example a U.S. B1 visa allows attending conferences and close business deals. But it doesn't explicitly allow working remotely for a non-US company.
In any case, in most countries whether explicit or implicit that distinction is probably the right one. Of course, no one is going to care or know if you check some emails and do some work--just like no one cared if you made some phone calls and did some work years ago.
But, in most places, you probably shouldn't show up and say you'll be spending the next 90 days working remotely.