Step-grandparents who send birthday cards from year one are making a choice nobody required. Notice it.

No obligation exists there. A step-grandparent has no history with you and nobody would have thought badly of them for sending nothing, which makes the card from the first year a decision rather than a duty. That is the version of family that gets chosen rather than inherited, and it is easy to miss because it arrives in a small envelope.

Send one back this year, on their birthday, by mail. Write a sentence with something specific in it rather than a signature. If you do not know when their birthday is, that is an easy thing to find out and worth doing.