I think this fear is not entirely wrong, but is significantly overblown. There's a significant gap between what job descriptions ask for and what companies will actually hire. A resume full of dead technology isn't going to help you ladder your way up to bigger and higher-paying roles, but it's also not going to make you unemployable.
A resume full of less-shiny but still relevant technologies, otoh, has a lot of potential. You have fewer places to look for jobs, but you're a lot more exciting to the places which do want your skills.
When I look for a new position, I try and make sure it’ll allow me to learn a new fundamental skill rather than the latest new framework (though often there’s overlap:)
Over 30 years that’s allowed me to be employable across a lot of different tech stacks.
A resume full of less-shiny but still relevant technologies, otoh, has a lot of potential. You have fewer places to look for jobs, but you're a lot more exciting to the places which do want your skills.