It is part of the Global Climate Strike, globalclimatestrike.net/.
It is not a paid advertisement. I can't comment on whether DEV.to would do it for anything else, or what their guidelines might be. But for this instance it seems that they understand the impact and importance of climate change. As many organisations and companies are doing today – See uk.lush.com/ and specialized.com/gb/en (while neither of these have the same overlay, their reasons are the same).
I doubt DEV.to plan to do this as a normal thing (and I hope not), I see this as more of a one off. It would be annoying and it would dilute the importance of the message for the Climate Strike to start doing it for other things.
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