There are a lot of journals with their own editorial boards and these mass-resignations happen somewhat regularly. My guess is that usually some people on the board will first want to decide to get away and then figure out where to move to (eg find some other publisher that is more preferable / get some funding from somewhere for a cheaper to run journal[1]) and then maybe also figure out a plan for the new journal to have a good start (eg with some good papers ready from people who are less constrained by journal impact metrics). So these things tend to take a long time to happen and aren’t really precipitated by EU rule changes.
[1] In mathematics there is the concept of an ‘arxiv overlay journal’ which is an example of something cheap to run: you have a website and editors and forewords and suchlike but each issue is, in some sense, a list of links to specific versions of papers on the arxiv.
[1] In mathematics there is the concept of an ‘arxiv overlay journal’ which is an example of something cheap to run: you have a website and editors and forewords and suchlike but each issue is, in some sense, a list of links to specific versions of papers on the arxiv.